Thanks Michael.

I've switched my session cache to file based and the problem has 
disappeared (which resolves my issue so if you want to close this 
ticket feel free), so it's definitely related to storing session data 
in the db.

I tried bumping the settings in mysql but the problem persists.

Here's my mysql settings:

key_buffer_size = 256M
max_allowed_packet = 1M
table_open_cache = 256
sort_buffer_size = 1M
read_buffer_size = 1M
read_rnd_buffer_size = 4M
myisam_sort_buffer_size = 64M
thread_cache_size = 8
query_cache_size= 16M
thread_concurrency = 8

user            = mysql
basedir         = /usr
datadir         = /var/lib/mysql
tmpdir          = /tmp
language        = /usr/share/mysql/english
skip-external-locking

Your session storage is most likely too small - your SQL server is 
probably limiting data entries to 64 KB.



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I have a user with > 300 IMAP folders. When he clicks Folder 
Actions->Show All Folders, then tries to select a message from the 
message list, he is dropped out to the login screen. Message on the 
login screen after being kicked says "Your session has expired. Please 
login again."

I was able to reproduce the same behavior by creating a sub-folder 
called test with 300 subfolders below it. The problem seems to be 
associated with the total number of folders in the account, not an 
individual subfolder.

I'm running Horde-4.0.14, IMP-5.0.20. Backend is Cyrus IMAP. Session 
handler is SQL Database (MySQL).

Am I missing a setting in php.ini? I've tried bumping memory_limit (as 
far as 512M), post_max_size, etc to no avail.