As you have probably noticed, RMU seems to regularly fail it's licence check, so I have tweaked the check to add some re-attempts in order to not have to close and re-open the project

In "Assets\RPGMaker\Codebase\Editor\Common\RPGMakerEditor.cs" go to Line 130

Replace 

private static async Task AuthRpgMaker(){...}

 With

 

private static async Task AuthRpgMaker()         {             var attemptCount = 0;            while (attemptCount < 3)             {                 // 認証                 switch (await Auth.AttemptToAuthenticate())                 {                     case Auth.AuthStatus.AuthenticatedByUnityAssetStore:                         // Unity Asset Store認証成功                         return;                     case Auth.AuthStatus.NotAuthenticated:                         attemptCount++;                         AuthErrorWindow.ShowWindow(EditorLocalize.LocalizeText("WORD_5014"),                             EditorLocalize.LocalizeText("WORD_5011"));                         if(attemptCount < 3) continue;                         throw new Exception(EditorLocalize.LocalizeText("WORD_5011"));                     case Auth.AuthStatus.NotAuthenticatedWithConnectionError:                         attemptCount++;                         AuthErrorWindow.ShowWindow(EditorLocalize.LocalizeText("WORD_5014"),                             EditorLocalize.LocalizeText("WORD_5011"));                         if (attemptCount < 3) continue;                         throw new Exception(EditorLocalize.LocalizeText("WORD_5012"));                     default:                         if (attemptCount < 3) continue;                         throw new ArgumentOutOfRangeException();                 }             }            try             {                 Migration.Migrate();             }             catch (Exception)             {                 // Handle exception             }         }

This will force Unity to reattempt the licence check up to 3 times before showing the licence error.

This is not a 100% fix or work around, but should hopefully reduce how often you see the error.