Hi,
Hope you are doing good.
The error you are facing is described in detail in: SAP Note No. 1659382 - After entering correct user ID and password the logon page is displayed without an error message. Please do have a look.
We have seen similar cases in the past due to wrong logon modules configuration. Make sure that configuratoins are as per:
If the issue persists, run the diag tool as outlined in SAP Note No. 957666 and reproduce the issue (login to the portal) and the output should help.
Hope this helps.
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Kind Regards,
Hemanth