Thank you for the details of the issue you are experiencing with the Mac App Store. I recommend the following article for the issue you described (the title of this article references an error that you didn’t mention, but the steps are relevant for the issue you described):
Thank you for using Apple Support Communities.
Thank you for the additional information. Due to the nature of your issue or question you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action
All the best,