I haven't seen it, but things that come to mind to narrow it down:
1. Did you possibly enable TLS on the Prisma DICOM node you set up for your XNAT? XNAT can't do TLS. Any other settings you changed on the Prisma for your XNAT?
2. Make sure you are able to send to the XNAT DICOM receiver on your XNAT from any other tool -- dcmtk dcmsend, for example?
3. Can you ping your development XNAT's IP address from a command prompt on the Prisma console? Maybe there were network changes you weren't aware of..