It's a good next step I think, yes.
No idea 
last guess, what is subject and identity in the output of mount webdav command:
issuer: Let's Encrypt, US
subject: fairlabwebdav.ddns.net
identity: fairlab.uk
fingerprint: ***************************************
Look at the man page for davfs2.conf and edit it in /etc or in /etc/davfs2 or something? If you've added the trust_ca_cert you can probably comment that back out to get it to fall back to the OS certificate store.
FOUND IT! 
it was in my local .davfs2 directory and the path to this certificate was in the davfs2.conf file.
Osiris, Schoen thank you so much for your help I wouldn't know where to start without you.