11.8 is the oldest shipping release right now, and it was compatible with 9.1+ in the backend, 11.9 and later need 10.5 as minimum.
Someone recently had some issues with Jabber and had to downgrade to a version which was compatible with their CUCM/IM&P.
You're running a not supported combo of Jabber and CUCM/IM&P releases.
I'd suggest you review CUCM traces and Jabber PRT and see if something stands out, but if you wish to use the latest Jabber releases, you need to upgrade the backend as well.
if this helps, please rate