Manually unflag a user who's flagged as abusive

Lennert Fransen Just to be sure, have you checked all three tabs in the Moderation menu to make sure the member isn't marked there? Is the user a member of any roles that would qualify them as "abusive?" Are there any similarities in the content being posted by them, maybe their signature has an "abusive" term in it that is triggering the moderation? Does their user name possibly have an "abusive term" in it (for example, the middle three letters in my last name have caused me to be automatically flagged in the past )?

Lennert Fransen Just to be sure, have you checked all three tabs in the Moderation menu to make sure the member isn't marked there? Is the user a member of any roles that would qualify them as "abusive?" Are there any similarities in the content being posted by them, maybe their signature has an "abusive" term in it that is triggering the moderation? Does their user name possibly have an "abusive term" in it (for example, the middle three letters in my last name have caused me to be automatically flagged in the past )?

Alex Nassi , yes I did, the user isn't showing up in any of the thress moderation tabs anymore, all posts are approved, even the one that was identified as abusive. 

All the other posibilities you mention are also not related to this case unfortunately. In this case, which was us testing some things, we posted a question, which we then marked as abusive with another account. The post got into the moderation queue and the user was flagged as abusive automatically by the system. Which is all correct. 

We then approved the post and made sure nothing was still in the moderation queue. Then we tried posting again, which was not possible anymore as the "user is identified as abusive". 

I would assume that when the posts are approved and there's nothing else 'wrong' with the user, he/she should be alllowed to post again but the user remains flagged as abusive for a certain time

Approving the content is not enough, you must also go in and approve the user who if is marked as abusive will appear on the appropriate tab in moderation.  If you are seeing something differently I suggest opening a support case.