Most likely the billing module release and ispconfig version dont match. Which billing module revision and which ispconfig version do you have installed?
The billing module rev is not comaptble with ISPcobfig 3.0.5.x, the rev1 is for ISPConfig 3.0.4.x only. Please conatct me by email to info [at] ispcofig [org] so I can send you the updated version for ISPConfig 3.0.5.
oops. I found the email from you with ISPConfig billing module 1.3 from February. I forgot to update... After the update is not a problem with billing. Sorry for my stupid question and thank you very much.