Roundcube session times out while performing actions in webmail

Symptoms

When attempting to send an email via webmail, you will see "Sending Message..." for several minutes, and the message will fail to send. You may also see Server Errors pop up while navigating through webmail.

Cause 

A broken webmail signature causes this issue. In this case, the signature contains an outdated webmail session ID (highlighted by cpsess##########) in the image URL. You can inspect the image by right-clicking it, then navigating to View Selection Source in Firefox. In Chrome, you can right-click the image and select Inspect to inspect the URL. 

Resolution

Remove the broken images/links from your webmail signature. 

To edit your signature in Roundcube, you can navigate to Settings / Identities for the user in question and make necessary changes.