While official support for the Larry skin was deprecated with the release of Roundcube 1.6.0 it is still available both via composer and directly from github.

not helpful - if you look at the forum posts of the last 2 weeks, I'm not the only one who has one or more problems with the new skin.
It would be kind of nice if the developer would speak up about what he was thinking and take feedback to restore the readability in the mailbox... 🙄

I posted this issue on Roundcubes Git as well, and someone there posted this: https://github.com/roundcube/roundcubemail/issues/9436#issuecomment-2097345467
It seems like the latest version of Roundcube is the culprit.

yah it's me ..Yes, the issue is from Roundcube

A similar comment was posted earlier in this thread. I do not use cPanel or Cloudflare. Based on posts about other cPanel powered systems I believe cPanel adds their own plugins like cpanellogin and cpanellogout for example to the Roundcube. The suggested "fix" would, I guess, remove the cPanel plugins from the application as it says Roundcube was installed from source rather than through cPanel. So my question is, have you tested a 1.6.6 install of Roundcube without the cPanel plugins?

I took the files from another website that also uses cPanel because I know cPanel has additional files within Roundcube. That's why I pulled an older version of Roundcube from a website using cPanel.
you can download my files to try it ..
https://github.com/roundcube/roundcubemail/files/15241442/roundcube.tar.gz

It could may be the 'x_frame_options' or 'session_samesite' config options you need to adjust. Details of the issue would help, what is reported in the browser (and browser console) or in the Roundcube error log?

iframes are often messy when it comes to browser security, the more info you can provide the more likely someone has some advice for you.

This seems to be a common question but I can't quite nail the answer. I have an i-frame on my site and it displays the domain's Roundcube webmail. That's all fine, but it won't allow the user to insert their password, rejecting the action as invalid.

Any easy fixes? Many thanks.

I have a installation of Roundcube 1.6.6 behind a Cloudflare proxy and it works fine. I'm guessing it is aa issue with the cPanel customizations or web server config that is causing the problem.

I have 8 dedicated servers, and all the websites hosted on them are experiencing the same issue. Two of these servers were installed just 4 days ago without any modifications to cPanel. I only installed the system, cPanel, and transferred the websites to them without NS or anything only change DNS from cloudflare, yet the same problem.

But I followed the steps I mentioned above and solved the issue on all of my servers.
It's a temporary solution until roundcube and cpanel fix this issue.

Just so you know, when the issue was present, I created a subdomain in Cloudflare for another domain and linked the IP address of the server experiencing the problem. This allowed the webmail to work for me.

Example:
The problem was with this domain, "webmail.server1.com."
I added a subdomain on the domain "test.server2.com" and linked the IP address of server1 to the subdomain "test.server2."
This made the webmail work for me.