Nicolas,
Thanks for all you help on this one. I'll add this section even though you sorted it out privately since there may be others with the same issues with Trixbox 2.8.x
/etc/asterisk/manager.conf has the recommended lines of:
read=system,call,log,verbose,command,agent,user,originate
write=system,call,log,verbose,command,agent,user,originate
Changing this to:
read=all
write=all
solved the problem.
Many thanks for all your hard work.
Carlos.