Hello!
Not sure if this has any connection with the Protinus, but German Naval Staff war diary (English translation!) for March 20th, 1940 writes:
(North Sea)
"
Air reconnaissance led to various brushes with British trawlers on the Dogger Bank. One trawler was obviously sunk by a hit in its vicinity."
See:
https://archive.org/stream/wardiaryg...e/126/mode/2up
The war diary is not clear when the incident happened. May have been late 19th March, too? Original German diary may be clearer.
The diary also describes the RAF attacks on 19th March.
Cheers,
Kari