I'm getting the same "Invalid XML" error message and I've tried all but one thing to get it resolved: Setting the Global ID as the Primary Key on all of the relationship classes as suggested in the Prepare Data For Offline Use documentation. I hesitate to do it because this is our client's database and it works for them on their collection devices. I'd really like to see this issue resolved going forward or make it well-known that if you ever intend on using the Collector app to capture data offline, that Global IDs are the only way to establish relationship classes. I added a proposal on the Ideas page to prompt the addition of Global IDs before using Create Attachments (Create Attachments - Prompt for Global IDs). That might alleviate some of the issues.
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