I am facing the same problem. I had one ssl certificate setup for the following domains:

firstdomain.com
*.firstdomain.com
a.firstdomain.com
b.firstdomain.com
a.seconddomain.com
b.seconddomain.com

When i read that it could be due to multiple different domains using the same ssl certificate, i removed the seconddomain.com server blocks from my nginx. So now i am left with:

firstdomain.com
*.firstdomain.com
a.firstdomain.com
b.firstdomain.com

The error frequency seems to have gone down but i still see some errors. How do you suggest i solve it?

I can try and merge a.firstdomain.com and b.subdomain.com so that they can be accessed from the same server block *.firstdomain.com

Then only 2 server blocks will be left:

firstdomain.com
*.firstdomain.com

Do you think i'll still face the same issue after this? If yes, should i move firstdomain.com to a different server?

If no, how do you suggest i try to solve this?

It would be great if you can help me solve this. Thanks.