The rejected share issue should be gone now. The message of failed to validate solution is because the pool has form a new job before the shares from the miner arrive. It's expected but it will be nice to have a better message.
That can't be true, because running 12.0.1 gets no rejects in the middle of a job (only at the same time of a new job) whereas 12.1.0 and 12.2.0 have tons of rejects right int he middle of jobs. About 2% difference in rejected share between the two, consistently. Mining on f2pool.