I cannot reproduce this problem here, can you attach an example file for testing ?

Step/Pic 1: Situation ...
Step/Pic 2: Block twitched to invisible, rectangle selection lower left to upper right,
Step/Pic 3: Click on copy (clipboard) ... warning pops up ....

Rectangle selection from upper right to lower left works fine!

Thanks, I understand you correctly and I see what you mean now.
Does seem to be confusing -unless there is an explanation for this (a bug..?)
The only way I've found to avoid the warning currently is to first select all with window left to right, then make the block invisible...then the copy option works out ok!

It's only the warning window - everything else works as expected.

Yes agreed everything else works as expected!

@Andrew, I agree with Husky here -unless we are both missing something and the warning is for something, but not working ??? otherwise we don't need the warning!

Additional!
Tool - Select rectangular area, lower left upper right, will also select invisible blocks in that area. If I use afterwards the delete function .... the invisible blocks are deleted too :-(

Upper right to lower left selection works as expected what means: Invisible blocks are invisible ;-)

This should be fixed for the next release (invisible blocks are not selectable).
Please note that a block might contain another invisible block, in which case it can be selected. However, when trying to copy it, a more generic warning appears:
"Transaction failed. Please check for block recursions and locked or invisible layers or blocks."

Sounds good - I will check it out with the next release. Thanks!