Fractional value in the workday (Timetracking settings) not well supported by GreenHopper

We are getting an Error Message in Greenhopper when trying to log work "Unable to log work". I cant find anything about this error message anywhere on the web, Logging work through the normal Jira interface and through the TEMPO Plugin works. What is the issue in Greenhopper? Is there a way to find out were and why this error gets thrown ?

WARNING: Parameters: Invalid chunk ignored.
2010-06-03 16:37:10,219 TP-Processor9 ERROR agrunewald 997x37765x1 1pclvj0 10.125.1.14 /secure/GHLogWork.jspa [greenhopper.jira.actions.CardBoardAction] java.lang.NumberFormatException: For input string: "7.5"

It seems as if Greenhopper is doing something with our hours per workday. Probably to parse how many hours 1d is. We set up 7.5h for our workday. Jira 4.1 supports fractions in the workday and I would assume Greenhopper has problems with that. I know that this feature is new for Jira 4.1