This message most often pops up when a script step attempts an action that the current user's access privileges do not permit. You'll need to take a careful look at any script that might be running (any script triggers on your layout?) at the time this error message is popped up.
I accidentally changed a field while in read-only, and I couldn't do anything (but was rather blocked by this error). I got around it by figuring out what data had been in the modified field and entering that back in.