Hello Yan and everyone,
It has been a while and this problem is still not solved. I have followed the steps you mentioned in this post (ODK Aggregate Keeps Refreshing) and in this GitHub entry (https://github.com/opendatakit/opendatakit/wiki/Aggregate-AppEngine-Troubleshooting#repairing-the-form-definition-table), but the same error ocurrs.
I hope somebody can shed some light to the problem here.
Best