Errors with UnityAds

I have successfully used UnityAds in a test application and also this particular game. However, once I created a release version of the game, the UnityAds seem too fail. Switching back to a debug release for some reason also did not work. Any idea what could be causing the errors below?

I am getting the following errors in my logcat:

2021-01-03 20:43:58.357 4168-4332/? E/UnityAds: (line:55) :: Error handling invocation[]): null

2021-01-03 20:44:57.134 4168-4258/? E/UnityAds:$d.a() (line:344) :: Unity Ads WebApp creation failed!

2021-01-03 20:44:57.135 4168-4258/? E/UnityAds:$e.a() (line:383) :: Unity Ads init: halting init in create webapp: Creation of WebApp failed!

If you can’t load advertisements only in publishing mode, it may be that unityads has no advertisements to display

I was e-mailed by someone from Unity that there was an issue with my proguard rules as below.

I’ve tested both builds and looks like you have an issue with the Proguard.
You can see in the documentation, that Proguard only works with Release builds (i.e cocos run -m release) debug builds do not invoke Proguard rules, that’s why your debug build is working properly.

I was advised to put the following lines in the Proguard.

The line with -keep class** seems to give me problems and I am getting the following error:

Do you have any advice for this error?

By the way, I put these lines in the files under the Gradle Scripts.