Sandboxie itself does not hide itself from sandboxed applications. Maybe the creator(s) of the application don't want to target Sandboxie, so you could ask if they can allow it again. Often, virtual machine (VM) detection also triggers for Sandboxie, erroneously. If they don't care or consider it a way to enforce a license, in other words a form of DRM, then it's their choice and David has stated that he does not want to evade detection as that will only end in an arms race.
You could try to use
SbieHide. Sandboxie-Plus has an addon manager, that lets you download and enable it. You can also download it manually, place the files where you want them and have the Sandboxie.ini point to the DLLs. The readme tells you what you have to do.
Edit: As for licensing questions, please send an e-mail to David, if there is an issue with Sandboxie's mechanism to get a test certificate.