I've got a hosted feature layer that I set up in Arc Pro and published from there. I did it this way because it was my understanding that this is the way to create a hosted feature layer with a related table.
I created a feature class in the default .gdb for the Pro project, then created the stand alone table, created a relationship class between the two using the GlobalID field in both as the key. I then published using the 'Publish Web Layer' tool on the Share tab. In AGO, I made sure that the feature layer has editing enabled and that both that spatial layer and the table had attachments enabled. I added this to the web map driving my Collector project and proceeded from there.
In Collector, I am able to create the spatial feature without trouble. I then select the feature, scroll down to the 'RELATED' section and see the name of my related table in blue with the little link icon next to it. I tap on that, tap 'Add', fill out all 4 required fields and hit 'Submit'. I see 'In Progress' and then get the error message depicted here. "Unable to Submit 1 or more attributes failed"
I can't figure out where I'm going wrong. My googling and browsing here hasn't gotten me very far, but please forgive me if I'm missing something obvious.
Weirdly, when I turn on Logs in Collector, I don't see any entry for my attempt to submit the related record.
Please let me know if there's more information I should provide. Thanks in advance.