[TYPO3-project-seminars] Entering the Price in BE... always NaN ?

[Seminars [Typo3]]

Hi,

im using Typo3 4.2.1 and the current Seminars Extension for the first time.
On creating new seminars i want to enter the price but anytime the System 
replace the entered numbers like 123 or 123.12 or 123,12
and the system is setting the values to NaN....

so i couldn't enter any price... what is my problem and how could i resolve 
it?

thanks at all

hendrik