I'm going to go over a bunch of stuff that's kind of "common knowledge" now, but...
Do you have your synth rack open in the browser?
If so, is it docked at the top or bottom?
If it is docked on top, move it to the bottom; if that doesn't help, close it.
Does the "-" key in
Markers View work?
Did you "bind" the delete key in
Keyboard Shortcuts?
I had an issue with the delete key "bound" in
Shortcuts.
I would highlight a marker in the
Markers view, but whatever was selected track in track view is what got deleted.
The same "issue" in the
Event List, a "delete" there caused a delete in the track view.
That created some interesting/challenging issues!
FWIW, I've never had any luck trying to edit Markers from the timeline; just me maybe.
T