I bet it's too late to check wt5 settings, as it's been uninstalled now...
Anyway, other than the default TZ for webtop, users can setup their own TZ on the settings menu.
The user timezone is setup on first access by copying the default value above.
So, what was the TZ for the user? Maybe the default TZ was wrong for webtop, and users were created with that wrong default value? In this case, changing the default value later, did not update already created user's TZ.