My site (asp.net 2.0 c#) has 3 login fields, Group, User and Password. I use forms authentication with the following authentication section in my web.config :
Most of my users can login no problems, but for about 1 in 5 of them, they receive no error message at all and are thrown immediately back to log in page.
Every login attempt is written to a security log and according to that they passed my own inbuilt validation,
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