There is a clock test in the menu which will help you know if optos working. I'm not sure how easy it is to replace an opto since they are mounted on the boards. You can always try cleaning them. Look for guide taking clock apart. Nothing difficult but a few things are important. Easiest thing to do is upgrade to the german bd that has improvements to design. However sounds like it's difficult sourcing one with covid right now.

I had a "clock is broken" message at one time and it was the plug at the bottom of the clock board, it had a bad pin. The clock seemed to work thou. After fixing the plug it went away.

If you need a clock board I do have a spare I would part with.

The original Clock board design is flaky, repros with the same design have the same issues, believe me, I have tried. The only permanent solution is Ingo Kramer's Twilight Zone TZ Clock Board:

https://www.german-pinball-modular.de/final-tz-clock-pcb-english.html