I’ve been having the same issue (running DSM6.2.2) and just found the solution. The problem is with an expired root certificate as explained here: DST Root CA X3 Expiration (September 2021) - Let's Encrypt
The solution that I found is here: Updating CA root certificate bundle on Synology
Apparently as a new user I can only put 2 links in one post so I have to split my reply.