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Debian 11 is approaching its end-of-life (vendor EOL date - August 31, 2026). Plesk Obsidian 18.0.80 will be the last release to support it.
If you are running Plesk Obsidian on Debian 11, we recommend you upgrade those servers to Debian 12 using our dist-upgrade tool.
We plan to deprecate and remove the support for XML RPC protocol versions earlier than 1.6.9.1 in Plesk Obsidian 18.0.82. We strongly recommend that you update all existing integrations using earlier versions of the XML RPC protocol to comply with the version 1.6.9.1 specification.
It seems you need update your Plesk up to 7.5.4 and install 2 hotfixes available from sw-soft.com
Then you can try to remove webmail from IIS and then reconfigure it with "websrvmng.exe --reconfigure-webmail" from %plesk_bin%
If this will not help then you can modify Plesk installation via add/remove programs and uninstall Horde. Then delete folder webmail from vhosts folder and reinstall Horde via add/remove programs
This is a late response but I have had exactly the same problem and here is my solution.
webmail.domain.com, gave me exactly the same error.
when I tried webmail.domain.com/imp , I have found out that everything works fine.
Then I changed the "home directory" of horde to "C:\Inetpub\vhosts\webmail\horde\IMP" (may differ in your setup)
(Start / All Programs / Administrative Tools / IIS Manager / Your Server Name / WebSites / Webmail(horde) right click / Properties / Home Directory / Local Path)
After that, I changed the "Local Path" back to the original address. ("C:\Inetpub\vhosts\webmail\horde")
Then everything was back to normal and working perfectly!
Seems like it is kinda simple confliction in registery or something like that.