Hi @ipluskal_1214
You're right, there haven't been any RPM changes... the error message is implying that ProGet is rejecting the data you're sending for some reason, "Unable to parse package header. The supplied package may be an invalid RPM file".
It might be a .NET5 issue, just based on the version numbers you told me. Since ProGet v5.3.12, the inedo/proget image is hosted using the .NET Core runtime. Previously, it was hosted using the Mono runtime. The inedo/progetmono image is available to v5.3.19.
Cheers,
Alana