Error: Permission denied to access property "target"

[Ed]

Dear All,

I had only two TW5s open in FireFox on my PC Windows 7.
Both 5.1.14
I had moved two tiddlers from one TW5 to the other and
the dreaded red Warningscreen pops up with:
"Error: Permission denied to access property "target""
Aargh

The target TW5 I had downloaded from the cloud where
I keep my back-ups

Restart PC.

OK, start all over again. Three tiddlers moved.
Again the Red Scare.
Flabbergasted. Putain.
What to do?
Thank you very much in advance for assistance.
Tschö, Edm.

[Birthe C]

This issue about drag and drop https://github.com/Jermolene/TiddlyWiki5/issues/2916