Hi Rob - Thanks for your patience on this. So far, we have identified a few causes of this error (one being the duplicate coded value domains that we've talked about here, and which ArcGIS Online has fixed for new data). We are still in the process of identifying and handling the various causes and find workarounds for them (without republishing) where possible.
The other cause we've noticed has to do with editing coded value domains while the map is open in Collector (and in that case, restarting the app is the workaround, as reloading the map isn't enough). We are still in the process of understanding the conditions required to hit this issue. Are you doing your domain editing on a hosted feature layer? If so, does the map open if you restart the app?
This is why I recommended
A support request is the right path forward. We have seen this for some different reasons, and you might be hitting one of those, and not the one that was fixed.
We aren't sure all the cases yet and don't want to send people down the wrong path if in fact their case doesn't meet the requirements of one that we know. When working with support, we get the details and information to reproduce the error the same way you are, and can use that to understand if it is a different case and what is being encountered.
Know that we are trying to understand the causes for this so that we can fix them, and thanks for helping us do so!
Esri Education Team
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