Thanks, that really helps!
Did you were tracking the beta branch at some point and then switched to stable? The private_message column was added in v2.6.0.beta2, but the commit is not included in v2.5.2 +2. This is the commit:
https://github.com/discourse/discourse/commit/92b7fe4c62094275c5dd09b1f3dac5c5cbcad1c0