[horde] Invalid Token for reopen mail as HTML or view source

[lst_hoe02 at kwsoft.de]

Hello,

we have a test install with Horde 6.0.0-RC7 and matching IMP. It works  
basically but we are not able to view mail source or the HTML part in  
a separate browser window. The error is "Invalid Token" and no content  
is displayed.
The syslog contains the following:
2026-06-22T11:57:40.217804+02:00 schulung HORDE: User xxxxxx is not  
authorized (Remote host: xx.xx.xx.xx) [pid 204604 on line 428 of  
"/var/www/horde/vendor/horde/core/lib/Horde/Registry.php"]

So it looks like a problem with authentication token?

Thanks

Andreas