Yes, most current Safari and Exploder. Firefox doesn't seem to mind, although it may be a couple versions behind.
Exploder says the error indicates the certificate presented was issued for a different website's address. Safari says there is a host name mismatch.
I noticed that the name and DNS fields on my other certificates have both" www.*****.com" and "*****.com" listed where Fastmail's contains only "******.com"
If I select visit site anyway, my browser trust settings get updated and I can then open the page without issue.
Anyway, I'm virtually clueless in this area and just report what I see.
Edit: The latest Firefox version does the same thing. The FireFox error message says the certificate is only valid for *****.com. not valid for
www.*****.com.
Error code is: SSL_ERROR_BAD_CERT_DOMAIN