(i use addons but i am not able to guess which one could generate this message)
This error means that function called from some .xml file is not defined. IMHO it is not possible to tell which addon it is from this message. Try to update all addons and if it does not help, you will have to disable addons one by one and find the culprit.
Thanks for the answer everyone, i will do that tomorrow, but that may not be easy, as this pops up very randomly and sometimes never during a full day.