The error about the UserConfig.xml seems to 'normal' for my server, so ignore that part.

Since the log also stated it did a update to build 5858 last night, and right after that the problems occurred, I did a rollback of the previous build (http://www.xeams.com/xeams-wont-start-after-update.htm) bringing me back to build 5849

Unfortunately, this didn't resolved the error. Somehow Xeams thinks it should talk TLS to the inside SMTP server, even these settings are disabled on Xeams nor did I ever used it (the inside server is not configured for TLS). I do believe it has something to do with the build update (5858) from last night, because right after that it stopped working (it eve wasn't able to send the update notification mail).