Code=134110 - Validation error missing attribute values on mandatory destination attribute

You've pretty much hit the nail on the head but it sounds like maybe you don't know why. It's because:

1. The attribute was required
2. Which means it must have a value when changes are saved
3. Migration saves changes, but
4. You didn't provide any value for this attribute.

That leads directly to the error that you received.

You can fix this using any one of the following:

• Make the attribute optional, as you did. After migration, no migrated objects have a value, but that's OK.
• Keep it non-optional but provide a default value in the model editor. After migration, all migrated objects have the default value.
• Set up a non-lightweight migration and provide values when migration occurs. After migration, each migrated object has whatever value you provide during migration.