Prob not as every time I got 'access denied' it was due to wrong config, as I found out later.
I had either the wrong domain names, apache conf files or DNS errors.
It was Aufs that stumped me in the end but you will prob find the errors are going into other_vhosts.log if the SNI & DNS is not correct.
I also split the log config into nextcloud.log and collaboro.log just to make things a little more easier to distinguish.
Also does your client and server have the correct DNS for the IP running example.com?
example.com should be your standard nextcloud domain.
The second domain often 'office' is just using apache (Server Name Indication) to proxy to 9980
Somewhere along the line your config isn't right, but without seeing whats going on in the logs its hard to say.
ps the original
docker run -t -d -p 127.0.0.1:9980:9980 -e 'domain=cloud\\.nextcloud\\.com' --restart always --cap-add MKNOD collabora/code
docker run -t -d -p 9980:9980 -e "domain=example.com:51111" --cap-add MKNOD collabora/code
should look like
docker run -t -d -p 9980:9980 -e "domain=example\\.com:51111" --cap-add MKNOD collabora/code
But like I say you should be getting log info to back this up.
I would go back to https://nextcloud.com/collaboraonline/ as there have been a few apache.conf and docker changes that are only slight but might well be the prob.