What the **** I thought, let's give auto update a try. With my new hosting provider creating daily DB and file backups, I might as well.
The update stranded with and error: incorrect file permissions on the install folder (must be higher than 644 and lower than 755) and some restrictions in .htaccess.
After that, the splash was the same (incorrect DB version) as reported by lefttees: http://forums.b2evolution.net/error-after-installing-blog-to-my-website-help
while adding an underscore on umaintenance.html did not take the site out of maintenance mode.

I love experimenting when I know I'm backed up, and I don't know where the error message originates, but it might be helpful for B2evo to indicate what file permissions must be set on the install folder - I tried 600, 644 and 744 to no avail - while also what .htacces rules must be modified --- that might be trickier ;)
I bet FP is going to say I must read up again, but auto update is in place since v4 - at least - and one would expect it to work without mayor hiccups by now.
That said, everything went smooth until this happend, so a few instructions at this point might just be the ticket.
Anyway, just beta testing again, hahaha