Ubuntu seafile server ssl issue


I’m running seafile server under Ubuntu 16.04 with Apache for a couple of years.
For HTTPS i followed the guide and used certbot.

Lately i have connection issues into my own domain, no connection possible for a certain period of time.
Sometimes it works and i can connect other times it seems the connection is down.
I can always connect with the internal IP address using HTTPS.

I’ve been looking at the Apache log and Seafile log, but cannot see any issue.
I’ve replaced my router and set port forwarding again, but this didn’t help much.

Does anybody have an idea how to debug this problem?
I guess it has something to do with the SSl certificate, but i’m not sure

any tip is appreciated,