I was able to replicate this as well, thanks for the steps.

I will add this to our bugs list, shouldn't be too crazy to fix.

Just a reminder that this is a fairly significant bug and it still exists in 15.0.5962. Thanks!

I can verify that the first part is an issue from within webmail.  And I logged out and back in before renaming "test" to "test3" and the problem is still there. This isn't an issue with just appending numbers; it also happens if a space + characters is added to the folder name.  Did not try the delete operation using IMAP though. The deletion part seems to be OK In webmail.

 

You also cannot rename a folder to simply change its case, e.g reports -> Reports

 

It tells you the folder already exists

We fixed the original issue that Kevin D. reported, in regards to renaming folder with similar names.  It will be included in our next 15 minor.  We're still looking at the other issues that have been reported on this thread.