I submit a support ticket already but as I'm waiting I'm going to see if the community at large has had this issue too.

I upgraded my community to 4.0.1 last night and everything seems to be working fine (except for a few things that I also submitted about but these have to do with Pages, not the forum) and now I can't post new topics.

I can reply, just not create a new topic.

Things I've already done:
Verified Permissions
Checked for errors in the logs (it's not generating an error at all).
Verified that it's not just my account (tried from a member account as well as a moderator account).
Went through IPS troubleshooter
I only have one third party application at the moment and that one was not installed until after I started to have this issue.
Verified default skin does the same thing.
 

If you've had this problem and had it fixed let me know how to do it so my members can start posting again! Thank you!

do you have plugins/add-ons installed?

probably they are causing problems with the new upgrade

 

As I stated in the original post, no plugins and this issue still existed. I did add one but the issue existed before that. Thank you though.

Figured it out. Appears it was old files on the server from 3.4.7. I removed most (except for the Pages uploads) of them and that appears to have fixed it.

Support can close this. Thanks everyone!