Because I have an old debian distro with a openvpn server with 2.3.x version that cannot be upgraded, I faced the same issue with an 4G-AX56 asus router.
After several config tries, and looking to the server log it appears that the credentials were not provided although those were in the profile form filled in the AX56 router admin portal. It seems that providing the credentials this way in the profile file and specifying those credentials also in the AX56 form of the admin portal solves the issue:
;auth-user-pass blob.txt
<auth-user-pass>
myuser
mypassword
</auth-user-pass>
Not sure it's gonna help others but who knows? ...