I can confirm that I haven’t come across this error again! 
However, I think this issue was mostly due to my browser cache, and from pressing “Run Online” directly from PsychoPy (which runs the experiment in a normal browser window). I use a timestamp on the first page of my exp. to ensure the version I’m testing online is indeed up-to-date with the local version, and this has revealed the online version doesn’t always update properly. Chances are, PsychoPy / Pavlovia is working fine and your exp. is indeed syncing correctly, but your browser cache is the issue.
If anyone is having issues with syncing, and you’re confident your JS coding is correct - try running your experiment in an Incognito window - that’s where I do all my testing now, and it works every time.
- Make changes to your PsychoPy exp. locally >
- press Sync with web project (but don’t press run online) >
- paste your exp. URL in an Incognito tab.
Of course, this is only an issue for us researchers when we’re testing out our changes. Participants will only view the finished version of the experiment - once, presumably - and will not be returning to the same link after changes are made, so browser caches won’t be a problem there.