[imp] Session expired, after drag and drop attachment

[Mike Egglestone]

Hello,

Running Debian 10 with IMP 6.2.22.
I can successfully attach any file in the "New Message" window by  
clicking the Add Attachment link and I can browse to any local file on  
the computer. I can also insert an image (html compose mode) by  
clicking the image icon and upload an image file from my local  
computer into the body of the email.
But after drag and dropping any file into either the Add Attachment  
area or into the body of the email results in a quick logout of the  
user that falls back to a login screen with "Your session has expired.  
Please login again."
The entry from /var/log/syslog shows a horde entry that is the same as  
if the user simply clicked the logout icon.

I've tried on several Windows 10 browsers (FF, Chrome, Edge and IE)  
with the same results.
Could this be a problem with my horde/imp configuration?
Any ideas to help troubleshoot this would be most welcome.

FWIW, I can drag and drop images into my wordpress site on the same  
system, so perhaps it's not a php issue?

Thanks,
Mike