Login issue with Marcus Povey's Elgg multisite

Hi All,

We are a talent tech startup based in India and use your multisite elgg to manage the central feed and chat for different tenants (clients) through the same. 
Suddenly one fine day the multisite login credentials are not working and we get a whiite screen. I had an Elgg expert from the community look into it but since the documentation is limited he was not able to help.

Request your help to resolve this issue. 

Best / Linda