Thanks Horst, your tip for turning on Debug solved my problem. I did not know there was a Debug setting.
Debug gave me an extensive list instead of the short error message.
The problem came from having one file in the template directory named "!tmp.php".
Just a scrap file I used as clipboard for bits and pieces, named with a leading "!" so that it would sort at top of the file list in my editor.
Now, the "!" in the file name was the culprit. Naming or renaming any PHP file in the /template/ directory with a leading "!" will re-create the problem.
Lesson of the day: Do not use leading "!" in template file names.
Thanks everyone helping out in his forum.
Process Wire is just amazing.
Lars's post in Smart Widgets / Blocks was marked as the answer