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.
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 In this case, the signature contains an outdated webmail session ID (highlighted by cpsess##########) in the image URL.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.
The workaround to the issue would be to remove the broken images/links from your webmail signature. To edit your signature in RoundCube, you can navigate to Settings > Identities.