I've upgraded nearly all ISPConfig systems I maintain from 9->10->11, and I can't think of any particular failure in roundcube. I typically install things from debian packages when available, so have roundcube installed that way on most (maybe all?) of them, and they just upgrade along with the rest of the OS; did you install it from source or ?
I used this on LXC containers: https://www.howtoforge.com/tutorial...-9-stretch-apache-bind-dovecot-ispconfig-3-1/ I only tried one time to upgrade but since the updates have now stopped coming it's time to try again. Last time I tried Roundcube pretty much died.
I see several problems with the upgrade to Debian 10. (I'm not even trying to upgade straight to 11.) PHP upgrade fails to install similar set of newer PHP. I have to manually re-install the missing packeges. I have to manually install missing Dovecot packages. (Why did that fail?) Letsencrypt -> acme Roudncube config with ISPconfig plugin In short. Never in my 20 years with Debian has the upgrade been such a pain.
Actually, should test the OS version or read the tutorial before proceedings since some softwares might not be available or compatible in higher OS version via apt install. Plus you are using LXC and thus its version which may differ from full version, that to me will require more tests before upgrading.
Just install the roundcube package again. You may have to run through it's config afterwards, but there's only a few settings which should need changed.
