I see more users complaining about this issue, sorry about the repost...
I've been looking for the source of the problem.
The following widgets/files have no problems:
The following files are causing the trouble:
Renaming these files (or removing them) makes the widget page working again.
I'm not a coding expert, but when I look at the files I see that the non-working files end with the 'add_action' call in the widget registering part ('public function __construct()'.
The working files don't have this function call.
Maybe it has something to do with this?