On UCM most deployments now use the E.164 for directory numbering, eg +3227450445.
Seems the Jabber Guest server does not accept this format for the 'Destination' field which is rather strange to me.
Will this be supported in a later version?
This will most likely never be supported. However you can simply setup a transform on your Expressway-C or Expressway-E to add this if you would like or have it added on your CUCM.
Is it possible to add the hex value of the +-sign instead, e.g. %43 ?
Do you have E1.64 route pattern already in Place in UCM ?
If not, create correct route pattern such as \+32[2-9]XXXXXXXXX as an example.
Make sure not digit striping neither prefix should be use.
I have created lots of E1.64 route patterns in UCM and they work well with Jabber as well.