I do not know what to say. It needs to be a pkcs8 format key file, and given the file name it is clear you know that. And unless you converted it with the -nocrypt option you need to supply the key password, which you have shown (redacted) in your configuration.
It has come up a couple of times in the last week (probably because the bug about logging the error at debug level was introduced fairly recently). This user got it to work, this one did not (so far).
For testing purposes can you try converting the key with the -nocrypt option and removing
ssk_key_passphrase => "removed"
(I assume that is ssl in your configuration, not ssk, since the plugin would complain if you had a typo in the config.) In fact, you just need to find what it takes to get the plugin started, you do not need anything to connect to it, so you can generate a throw-away cert with nocrypt for the test.