In my case I was trying to read my cer file and was receiving the error stated above
openssl x509 -in CSR.csr -text -noout
unable to load certificate
140518720210760:error:0906D06C:PEM routines:PEM_read_bio:no start line:pem_lib.c:698:Expecting: TRUSTED CERTIFICATE
I had to convert it to a crt file using openssl.
openssl x509 -inform DER -in <certname>.cer -out <certname>.crt
openssl x509 -in <certname>.crt -text
Here’s the doc i used. I was able to read it using openssl after that