teraßyte

That error means at least one of your 3rd party applications or plugins is not compatible with PHP 8. Use RECOVERY MODE to disable them all:
 
Once you've done that login in ACP and see if you have any pending updates for your installed  modifications (there's a button you can use in the applications page).
After all updates are done look at your support page and see if you have any errors listed there that need to be fixed.
As a final step re-enable your modifications 1 by 1 and check each time if your site still works. Should the site stop working you have to go back and use recovery mode again, keep also note of which modification you enabled that made the site crash and take care of skipping it next time.

Looks like you have the converter application installed but didn't upload the updated files for it. When downloading the zip from your client area there is a checkbox you need to select in order to include the Convert application in the download.
Re-download the zip with the checkbox selected and then upload again the files to your server. Once you do that the errors should all go away.

The user is probably hitting the Total maximum storage limit setup for their group.
Let's say you limit a group to upload 10 MB and the user uploads 6 files large 1.5 MB each (for a total of 9MB). At this point the attachment field would show that your upload limit is 1 MB instead of 2 MB (because you've already used up 9 out of 10 MB available for the account).
 

Blog itself doesn't support paywalls, but maybe you can use the clubs feature and have them setup a paid club to access their content/blog.
 

That's probably it then. Try disabling it temporarily and see what happens.

Most likely you've reached the MAXIMUM VALUE of the data type used for the member_id column in the ipb__core_pfields_content table.
The current default value is BIGINT(20) but in the past it had a lower limit. The column was not updated properly in one of the upgrade steps probably.

You can use the Converter application to merge 2 Invision Community sites: https://invisioncommunity.com/services/convert/
It will merge everything though, not just select members and their topics. A workaround would be to make a copy of the site you want to merge, delete all the members/content you don't want to port over, and then do the merge with only the data left.

ACP > Community > FORUMS > Settings > Forum Settings TAB > Topic Summary Settings
 
There are 4 settings in that area where you can change how that summary in the sidebar is generated. By default it shows only after the topic reaches X pages, but there is a checkbox to make it always show which I believe is what you want.
 
Here's a link to the guide:
 

Looks like your server sometimes is not parsing properly the PHP code/files. You should contact your hosting about it.

They should really include the ROW_FORMAT=DYNAMIC in queries by default for such situations.
For now as a fix you'd need to change your Wamp server MySQL configuration to create new tables using DYNAMIC by default.
 
P.S.: You posted this in topic Marketplace Support. 😋

Go to the Manage Purchases page on your site. Click the Manage button for the package you want to upgrade. Below the Product Information box you'll see an Upgrade link. Click Upgrade and a page to select the product to upgrade to will load.

Upload the images in that same location, but instead of using the media tag just right click on the image you want to use and copy the image's link. Then use that in your HTML.

Viglink was removed in version 4.7.4 because the implementation doesn't work anymore: https://invisioncommunity.com/release-notes/
 
 

Those limitations only apply if you upload an avatar.

Try replacing the parent call (line 134) with this code instead:
# Get parent output (PHP 8 fix) $whosOnline = ''; if( \is_callable( 'parent::' . __FUNCTION__ ) ) { $whosOnline = \call_user_func_array( 'parent::' . __FUNCTION__, \func_get_args() ); }  

You need to update the permission for each Downloads category in your ACP. Remove the download permissions from any groups you don't want the button to show up for.

Based on the error it looks like your new server has STRICT MODE enabled for MySQL while the old server didn't. You can try asking your hosting to disable it to solve the problem.
 
That said, the error comes from a third party modification. Based on the name I'd say it's a modification that keeps track of who has viewed a topic. You should contact the modification's developer and report the bug.

Here's the culprit:
{{if $record->customFieldDisplayByKey('post-type', raw) == "video"}} Change it to:
{{if $record->customFieldDisplayByKey('post-type', 'raw') == "video"}}  

Yes, you need FTP to edit that file. Or any file manager included in your server's control panel (cPanel, Webmin, etc).

You can safely ignore that kind of log. It's mainly for developers. They're going to remove it in the next version (or the one after).

Which version are you using? As long as your server supports the webp format it should show, however there was a bug in 4.7.0 preventing it from showing in the list. The issue is fixed in version 4.7.1.

You can edit that setting in ACP > Community > FORUMS > Settings > Forum Settings TAB > Default forum view.

I guess the next step would be recounting the reputation then. Keep in mind that recounting the reputation will not consider deleted content.
 
The option for it is in a bit of a weird location:
Edit any random member in your ACP. Top-right you'll see REPUTATION LEVEL, click the arrow to open a dropdown menu and click Recount. A popup with 3 options will appear, click Recount for all members. Once the recount is done you may need to rebuild the leaderboard again. It the above still doesn't work there's most likely an issue with the conversion.

The best way is using the Requirements Checker:
 
 
Otherwise there's also a requirements.json file in /applications/core/data. This is the content for 4.7.1:
{ "php": { "required": "7.4.0", "recommended": "8.0.0" }, "mysql": { "required": "5.5.3", "recommended": "5.6.2" } }  

Both values are hardcoded in the php backend. You'd need a custom modification to change them.
Well, you could also edit the php file directly (as long as you're self-hosted) but you'd have to keep redoing the edit on each upgrade.