What permision should give to user to view application pages.

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SharePoint

Hi,
I am using sharepoint 2019 on prem

I have created group and assign contribute permission to it,
I am using application page where i have put custom web part to allow user's to upload documents on document library.
Problem:
as user login , he/she gets white page with progress image ( circle ), user can goto viewlsts.aspx and sitesettng.aspx using url but unable to view webparrt which is placed on application page ( its landing page )
Solution required:
What to do to solve this issue, user login to portal to find out landing page ( applicaion page with custom web part )
Note: I have not stopped/broken inherited permission..

What I have tried:

assigned permission to group to login, but hard luck as white screens comes up