eMClient keeps asking for Zoho imap password

Just switched to Zoho free email.  Setup went OK, Imap folders synced, but now eMClient keeps frequently/intermittently asking for Zoho Imap userid and password. Diagnostic runs OK, but when I got back to select an email to look at, the pop up windows asks for userid and password, prepopulated and the save password box ticked.

This worked without problem on Virginmedia servers, but I am moving from them to get round a different problem. 

I’m on latest eMClient V6, Win7 using MS Security Essentials and Malwarebytes.

Any suggestions would be very gratefully appreciated. Thanks.