SSL Certificate issue

Did you try to upload with different browsers?

There is a bug with Firefox.

Also what files extension are you using?

Thanks

I have tried the same with Firefox/ Chrome/ IE - all three web browsers.

Using Certificate (.cer format), private key (.key format) and also using the correct passkey, but still getting the same error.

Try different formats.
When exported from windows (full cert chain) to pfx,  import over here works fine. 

Try different formats.
When exported from windows (full cert chain) to pfx,  import over here works fine.