teraßyte

If I recall right, only accounts with ACP access are automatically excluded from promotion/demotion. There's no additional setting to exclude other specific groups.
In theory you could add them to a group with ACP access but totally restrict their account so they can't actually do anything (except changing their email address or password).

It's not really a question of when. Rather, do you need your users to be able to rate topics (from either 1 to 5 or 1 to 10 stars) on the forum?
For example I've seen some sites using the rating function for a forum where topics are actually reviews of products and people vote the topic based on the review's quality. Do you have something similar that requires enabling the rating option?
As for how it works: people can simply vote X stars but the voting itself doesn't really impact forum management. The only change is that you'd be able to order topics by total rating (or maybe filter by X stars; I can't recall right now).

I recall there was a patch related to dates for Pages. Do you have any patches listed in ACP > Support that need to be applied? If yes apply the patches and then try again.

Most likely you're not allowing guests to see the forum. Check the setting in "ACP > Members > MEMBERS > Groups > Guests > Edit (pencil icon) > Can access site?"

Edit the option Can login anonymously? for your groups in ACP. It has 3 choices:
Allowed Forced Disallowed If you're seeing everyone invisible this option might be set to Forced. If it's set to Allowed it might be a bug with the conversion that converted every member to hide their online status instead.
Or you can force everyone to be visible by choosing Disallowed so every member's choice doesn't apply.

Looking at your screenshot you have a suspicious autoDeleteMsg task that has never run and is not tied to any specific application/plugin either. My guess is that that task is always failing to run and is blocking everything else after it in the queue. That's why you keep getting the "tasks not running" message.

That's the standard download attachment link, but newer ones usually have a security key in them. Are the ones you can't download all uploaded after a certain date perhaps? If so, it may be because the security key is missing from the download link that you're getting the error. But as Jim M said, that's a custom modification so you should contact whoever made it for you.
If they're not available anymore you can look at the Providers list to find a new developer to take a look at it for you: 

Looks like this was also fixed in 4.7.0.

There's a single instance of it on line 71 in \applications\core\modules\front\search\search.php.
You can't just comment out the line though, otherwise you'll remove the meta tag for ALL search pages, not just /tags/.

When that option is enabled the topic is automatically locked and normal members can only vote in the poll without being able to post replies.

It's a known bug fixed for the upcoming 4.7 version: https://invisioncommunity.com/release-notes/
 

Does the application have any front modules? You need to click the lock icon on one of those modules to change group permissions.

That message appears only in ACP (for security reasons). The code will still be inserted properly on frontend in the location you setup.

Nah, I wasn't even thinking about Google (but it does help that you keep the 301 redirects active). I'm talking about the old links already posted. For example let's say someone posted a link to another topic in a post while on Xenforo (so it's using a Xenforo url format):
With the converter application enabled: it redirects this link to the converted topic on Invision Community and your members can still find the topic. With the converter application disabled/uninstalled: the link returns a 404 error and your members will have to manually search the topic (or someone has to replace the old Xenforo link with the new Invision Community link).  

Try using:
.ipsNavBar_primary #elNavSecondary_1 a:before  

As long as the semicolon is there that code works just fine inside a custom PHP block. I just tested myself by copy pasting your code (adding the semicolon) in a block. Then I added the block to a page.
Maybe you have more code in your block other than the one you posted in the first post?
 
And no, there is no need to even use a function. You can just input your PHP code inside the block without it. A custom PHP block with the code below prints the same output as your code above:
echo 'testing';

Add this css rule in your theme's custom.css file:
.ipsType_richText a { color: red; } Change the color to whatever you want.
 
This will change links in any kind of posted content, not only posts though. If you specifically want only topic posts it needs to apply to some more classes/attributes.

Looks like your server has only English locales available. Contact your hosting and ask them to add more of them. Specifically the ones you need for your language packs.

Most likely you set the forum to allow topic starters to mark as solved their topics and your admin group is a global moderator.
Edit your Moderators group in "ACP > Members > STAFF > Moderators > [Select Moderator member/group] > Edit > Forums TAB" and enable the option Can set best answer/solved for questions and topics? while also making sure they are assigned to the forum(s) the topics to mark as solved are in.

You can edit FURL rules in "ACP > System > SETTINGS > Advanced Configuration > Friendly URLs TAB".
For example you can change the topic view rule from:
https://domain.com/forums/topic/{#id}-{?} to:
https://domain.com/customname/topic/{#id}-{?}  
There are 8 rules in total you need to change to cover all the FURLs that contain /forums/.

The link can be found in the top menu: Help > PRODUCT ASSISTANCE > Providers => https://invisioncommunity.com/third-party/providers/
 
I'm on that list too. 😉

This one was fixed in 4.6.12 if I recall right. We can consider it solved.

What is the group ID? Most likely it's one of the default 3 groups created on install (Guest, Member, Admin) which cannot be deleted. The 3 lines below are from a conf_global.php file (group name => ID):
'guest_group' => 2, 'member_group' => 3, 'admin_group' => 4,  
Most likely the Active Member's group ID is 3.

Is that topic visible to guests? If not the icons won't appear because the services wouldn't be able to retrieve any data to share.

That's a different setting/area. You can disable that specific RSS feed in ACP > System > CONTENT DISCOVERY Streams > Activity Stream Settings BUTTON > Enable RSS feeds.