Saving draft boots back to site control panel

I'm trying to write a post and it seems to go fine until an unidentified point at which when I try to save the draft it boots me back to the site control panel.
I did see an error (once) which was ERR_BLOCKED_BY_XSS_AUDITOR which seemed to indicate a Chrome problem but when I tried on Firefox I had the same drop to control panel.

I was duplicating blocks then editing, so I tried retyping each element manually and saving after each but eventually the boot to control panel would happen again (seemingly without a particular reason).

Once the issue starts, when I reopen the draft I can no longer save or open preview without going back to the site control panel.
I have also tried opening a previously published post (in the editor) and tried to preview it without alteration with the same result.

I published the post and that worked, then went to unpublish and edit by adding a block. Stopped.
I also seem to be unable to upload images (upload failed). No idea if that is related.
Unpublished, and opened and it offered me a draft version. I think I selected to not use it and I was back editing. Publishing no problem, images uploading no problem.
Then back to the control panel.
Can't publish from the editor, but I can publish from the list of posts (green tick, red cross, etc).

Sorry for all the info, I'm hoping it will help!

I can't see anyone else having the same issue, but does this sound familiar to anyone?