Hi Thomas,
Thank you. Your guess was spot on!
I remember I read about the new functionality, but can't remember I touched that button (for sure I must have done that).
Luckily my TW was in Dropbox so there were backups and it was no problem to get the 'normal' situation back.
After that I stripped down the TW I thought it was corrupt; just curious why it happened.
I deleted plugins, normal tiddlers, overridden shadow tiddlers, added system tiddlers, state tiddlers, ...) but the behavior stayed the same. Only after I deleted configuration tiddlers I got the 'normal' behavior back but I didn't notice $:/config/TimestampDisable as 'suspicious' while that is the culprit.
Anyhow it is a 'dangerous' button since there is no feedback about the state. In my toolbar I color such toggle buttons when the state changes from normal to activated (see e.g.
http://tongerner.tiddlyspot.com/).
The good news is that all the RSODs I got with this TW (I think it is my oldest TW, started in the alpha phase of TW around september 2013) never could corrupt the TW in the real sense.
Thanks again, I will not forget this anymore!
Cheers,
Ton