Server 500 Error : Internal Server Error

Server 500 Error : Internal Server Error

My client has a very distributed virtual workforce who are using Zoho Sheets throughout the day. They all work in the same spreadsheet workbook but usually on different sheets.
 
They are receiving the following error quite a bit, I'm also getting the error.
 
Error 500
Internal Server Error
Kindly reopen the file and try again

 
It is making work almost impossible for some. Is there any resolution? I notice that in previous forum posts you suggested people visit http://a1.sheet.zoho.com , however if I visit this site it is no longer there.
 
Any suggestions on how to resolve this issue?
 
Glenn.