Sequence Line bug?

Hi all,

I've been working on an edit for a few days (and previously on other projects), but somehow with this project I have a strange error. I cannot adjust the blue line/dot inside the sequence, though I can move it back and forth in the preview monitor. The strangest thing, and making it impossible to edit.

Attached screenshot, where inside the red circle I drew a 'mouse', but if I press there, nothing happens (the blue dot only changes in the preview monitor). Thanks for reading and I hope someone has a solution! I tried restarting, new project etc (and copying the inside of the 'older' sequence, hoping it would work).