This just started for me today. I'm running devprev and updated it today before starting puzzles. I'm running multiple clients on other puzzles without this same problem occurring, so I think it's something to do with the puzzle itself.
not a suggestion, but possible bug. Don't know how to edit to say bug after already started feedback.
This just happened to me in the newest puzzle 628 as well, so maybe it's particular scripts. A third client running a different puzzle 627 did not have this problem, but all were running different scripts.
628 and 629 as well - seems to have started after the most recent update. Actions tab Wiggle and Mutate also seem to get stuck while trying to cancel. Cancelling works only up until about the 5th or 6th action then locks up. Only a full restart will clear the "cancelling…".
forgot to note in the first comment that I am running Win7 64bit. I need to read "how to make a good feedback msg"! sorry.
messages should be able to be edited.
Anyway, I'm having the same issue… it's the newest version of devprev.