Error only when not Debugging

Hi Everyxone!

I have a custom plug-In for Dynamics CRM 365 online and get a validation error:

"A validation error occurred. The value of 'justANumberField' on record of type 'entity' is outside the valid range."

To fix it, I tried to debug, but as soon as debug is active the error is not accurrring.

The PlugIn-Registration-Tool works fine, I am able to debug and go through the code step by step, but no exception :O

May anybody help and point me into right direction? I am new to this....

Thanks for reading