Hello Florian,
A private/public key file pair should be generated once (per person or organization depending on the situation) and then re-used regularly. If you have not used your old key pair before, and you are not sharing these with others, you can just generate a new pair. Just be aware that you need to import the public key into every R3 instance (admin panel / system / repository) where you want to import applications, signed by your private key.
But generally, you should be able to export applications with any RSA private key. Some common mistakes I´ve seen that might be relevant:
- Using the public key instead of the private key for export.
- Not including the descriptors, such as
-----BEGIN RSA PRIVATE KEY-----
. The entire key block must be used.
- Using encrypted private keys - this usually happens, if you generated keys for other purposes and encrypted the private key file. If you generated the keys in R3, this does not apply to you.
This is how it should look like: