As others have said, it really depends on the individual branch.
I opened an account with SCB in Hua Hin about 10yrs ago when on Tourist Visa. Very straight forward, just needed to show certificate of residence from immigration. I opened a 2nd account with SCB earlier this year in Sisaket on OA Visa. Again very straightforward with certificate of residence from immigration.
I just arrived in BKK on KLM. A bit of confusion in Schiphol getting all the Covid related paperwork checked but apart from that the flight was fine. Staff very friendly, good service, food OK (certainly had better) and hardly any passengers so plenty of room to relax.
I completed my COE yesterday and had no problems. As others have said, try a different browser. It might also be that your file is too large. I’ve had problems in the past where the site has timed out in which case just logging in again and repeating the upload worked for me.