So apparently this has happened in the past in different versions since I found the following thread with my exact same issue:
https://www.roundcubeforum.net/index.php/topic,23440.msg63509.html#msg63509I'll quote the first post:
"
Hi,
at one of our clients account the Inbox folder cant be displayed. He gets this message: "Server Error (OK)".
The Problem only occurs with the Inbox folder. Every other folder works fine, and in Thunderbird/Outlook we don't have any problem with the Inbox Folder.
I have this exact same problem only with the default email on a cpanel instance with a host. My cell phone's generic imap client can see the messages fine and all other folders and all other email accounts on the server work fine.
The host's response is something I don't believe:
"Webmail can't handle that much email in inbox, we see in your screenshot is it 289,161 emails." "Webmail is unable to handle that many emails in one single folder."
I don't think either of these is true because the inbox is still receiving emails and my cell phone client is able to see them. It is important to note that Horde also has the exact same access problem as roundcube.
From my roundcube about:
Roundcube Webmail 1.4.6
Installed plugins
Plugin Version License Source
archive 3.4 GPLv3+
calendar 3.5.3 AGPLv3 Download
carddav 3.0.3 GPL-2.0 Download
cpanellogin 11.58 cPanel License (Proprietary)
cpanellogout 11.58 cPanel License (Proprietary)
filesystem_attachments 1.0 GPLv3+
jqueryui 1.12.0 GPLv3+
libcalendaring 3.5.2 AGPLv3 Download
libkolab 3.5.3 AGPLv3 Download
return_to_webmail 11.88 cPanel License (Proprietary)
Any thoughts or ideas appreciated!