It seems like DA doesn't accept some private keys even though they are perfectly good for Apache.
I just tried to install a Geotrust certificate obtained via bulkregister and DA reported that it couldn't find a private key.
I then tried to install another certificate and it worked fine.
It seems like DA only accepts private keys which look like
-----BEGIN RSA PRIVATE KEY-----
bla-bla-bla
-----END RSA PRIVATE KEY-----
but doesn't accept those that look like
-----BEGIN PRIVATE KEY-----
bla-bla-bla
-----END PRIVATE KEY-----
As a workaround, I first installed cert. which was accepted by DA and then replaced the files inside /usr/local/directadmin/data/users/USERNAME/domains/ directory.
I just tried to install a Geotrust certificate obtained via bulkregister and DA reported that it couldn't find a private key.
I then tried to install another certificate and it worked fine.
It seems like DA only accepts private keys which look like
-----BEGIN RSA PRIVATE KEY-----
bla-bla-bla
-----END RSA PRIVATE KEY-----
but doesn't accept those that look like
-----BEGIN PRIVATE KEY-----
bla-bla-bla
-----END PRIVATE KEY-----
As a workaround, I first installed cert. which was accepted by DA and then replaced the files inside /usr/local/directadmin/data/users/USERNAME/domains/ directory.