I have recently tried to migrate my catalog with 115k pictures to LR CC. Migration went well except that only 66k photos are identified. No sync is going on anymore. So this is already a problem.
I opened LR Classic to see what I could do, but there I immediately got the following error message "A migration of this catalog to the LR mobile ecosystem appears to be in progress. It is strongly recommended that sync remains paused or off until the migration is complete. Otherwise, the results may be unexpected and confusing.", with 2 choices "Enable Sync anyway" and "Don't enable sync".
I chose to not enable sync obviously, and waited (for 2 weeks) to see if the migration would finish and error disappear. But same problem continues.
Any ideas ? Thanks !