Synolgy 918+ DSM 6.2.2 Update 4 cannot renew certificates

@mibeyer Two different LE certs expired the end of Sept. This has caused issues on various systems which use unusual cert validation or older systems not updated for several years.

I am not certain that you are missing an LE root of ISRG Root X1. And, if that is the cause of the problem I do not know Synology well enough to say how to add that to your system. Maybe a simple system software update would do it - I am not sure.

I searched the Synology forum a little but did not see anything exactly like what you see. But, there were enough similar comments that it could well be the cause of your problem.

A key change with LE ACME servers occurred on Sept 30 when they started using the "short chain" ending in ISRG Root X1 when before they used the "long chain" ending with DST Root CA X3. See this for more on these chains. Note that while the LE ACME server api uses the short chain, all the LE websites use the long chain for maximum compatibility (mostly with older Android devices).

You could try posting at the Synology forum. Maybe this extra info well help with that. This is the best I can do.