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It's hard to tell which version of the summary fields you are having issues with but I'm guessing it's 4.0.1 which was suppose to fix the exact same issue with 4.0.0.  It obviously doesn't. I filed another issue on github for the newest problem.  

https://github.com/progressivetech/net.ourpowerbase.sumfields/issues/45

I think the fix is to change line 4 in sumfields.php

From:  use CRM_Myextension_ExtensionUtil as E;

To:    use CRM_Sumfields_ExtensionUtil as E;