The provided certificate seems to be bad, thus it was not possible to replicate the issue.
Is it the actual certificate or edited on purpose?
I would like to get more details about your setup.
Is the issue specific to this one certificate or the error is displayed by trying to upload any certificate?
Does the same certificate work with other firmware versions and does not work with 7.3 only?
Does the router stop accepting certificate after update to 7.3 or do you perform some additional configuration changes?
What did you have configured, when your set up was working?
Did you update with Keep settings option enabled or disabled?
Could you provide troubleshoot files from a working configuration and after update, when you fail to upload the certificate? To generate the files, access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. Attach them by editing your question.
Would you agree for a remote session to demonstrate the issue and for us to look for possible causes?