The weather engine is from a group called Flight Xtensions. They havent released any product yet as far as I know, this is their first product. Its still in beta. I mailed them about it and they said it did not use FSUIPC tho.
None of the other menus in FS were hanging, at least not the ones I tried (I tried the Level-D menu under the Modules menu, it worked fine).
I unfortunately didnt think about checking FSUIPC.log, I disconnected from VATSIM, restarted FSX from the last autosaved location and finished my flight, altho this time without the weather engine running. The flight finished without any issues. The log file I have now is from that last flight.
I might try to fly that route again when I got more time later, so I can take a look in the FSUIPC.log file.
-Wim