Restart the computer. It sounds like Virtual Stuck Key Syndrome.

Have now restarted the computer but no, the problem remains.

Any other suggestion?

In that case the key (on the computer keyboard) could be physically stuck.

Since you get a “tr.” sign, I suspect that the letter T key is stuck.

Try tapping the T key repeatedly.

I use the mouse and icon, so no it wasn't the keyboard either. But I think I might have tracked something down...

In order to demonstrate for you I created a brand new sheet from scratch, filled in a few notes and used the tab slide tool and to my surprise it worked flawless. So I went back and opened the piece I had trouble with, tried a new tab slide but there I still got a "tr.~~~~". It appears that for this piece I made use of a template found on the net called "Bergmann Edition" with predefined symbols for Barre, guitar fingering etc. I've used it before without any trouble (but it might have been an earlier edition). Apparently this template redefines the tab slide tool. I wasn't aware this was even possible! Did you?

Thanks for your effort to help.

… Apparently this template redefines the tab slide tool. I wasn't aware this was even possible! Did you? …

It is an option in the Smart Shape Options.