Hi Joris,
Did you really didn't do anything else, like updating a module? Most of the time, these problems with Java occur because of duplicate files or different versions of the same library. Maybe Mx 7.11 included a Apache Commons library themselves. Could you verify which lib-files you have in the userlib folder which have Apache and Commons in its name? (like org.apache.commons.lang3.jar). Try to delete duplicates and then try again.
Hopefully this is the case!
Kind regards,
Johan