"Invalid transmission words 3,856,419", what does that mean?

It means that port has has seen 3,856,419 transmission words that were invalid since the counter was reset.

The important thing to check is if these are climbing. Invalid transmission words can rack up very quickly when you plug/unplug a cable until word sync is established. If the link is up and has not been touched and they are still climbing, you may have a problem.

A transmission word is a 40-bit group consisting of four 10-bit transmission characters (see 8b/10b encoding if this doesn't make sense). These transmission characters are encoded according to the rules of the 8b/10b algorithm, and to put it very simply, there are only certain combinations of transmission characters that are valid.

So when the switch detects a TW as invalid, the cause is a 8b/10b code violation. The root cause can be a faulty cable, faulty optics, basically anything on the link that could cause a bit to be screwed up along the way. But like I said, it would be normal to see these when a cable is plugged/unplugged, but abnormal to see them continue to climb with a stable link.

Since your switch is reporting these (i.e. your switch is RECEIVING TW's that are invalid), that means the link to check out is the one the switch is receiving (i.e. the Tx side of the HBA or whatever is connected to your port).

If you don't have a cable tester available to certify the cable, and the errors continue to climb, I would start with replacing the cable first. After that I would go for the optics on both ends. But also keep in mind patch panels or other connections in the path.