Would be interesting to know how this was fixed.
After the upgrade to IPS 4.5.0 failed on my live installation (with over 150,000 topics) and I was a bit too impatient, I decided to return to IPS 4.4.10 with a fresh installation and thus I deleted everything. After re-uploading the database back-up (version before I tried to upgrade to 4.5.0), I had the same problem as mentioned above on my live installation then. Thus I deleted everything again and have now a test installation on localhost. The problem remains there, too.
Any suggestions?