iPhone App will no longer connect - ssl error (PC / Web App connects ok)

I have been running rocket.chat server using mobile apps (iphone), PC app and web interface for 18 months or so without issue. As of yesterday I could no longer connect to the server with the iPhone app, instead I received an “oops can’t establish a connection with server, ssl error” however my PC app and web access will connect fine. Let’s Encrypt ssl certificate does not expire until November. Certificate is installed on Netgate security appliance which runs HA Proxy. Interestingly, it appears a database migration took place on the same day (30th September), although I’m not sure if that’s just a coincidence or not. Any suggestions on how to resolve the iPhone connection issue would be much appreciated.
rocketchat

I strongly suggest you check this:

You may need to upgrade your server.

There are more and more problems with the new iPhones. With each new release of the phones, there are more and more malfunctions. My most satisfactory phone was the 6 iPhone, and the following models have had more and more problems. The last of them - the 12 iPhone, locked spontaneously, and later talking on a forum with owners of similar devices, I learned that the iPhone often gets locked. It is sometimes impossible to fix it because the owner loses access to their accounts. Although some of the owners go to the service center, they often refuse to unlock the phone.