My users say that Safari doesnt remember them as long as before (logging in). Is this know issue?

I use Safari and do not have that issue.  All my members experience problems with login when Memcached is enabled.  Do you have caching enabled?

I have only "Cache page output for guests 10s"

Uploading this file fixed it for me:

http://community.invisionpower.com/4docs/knowledgebase/407-sign-in-requires-two-attempts-r134/

Thanks @Ralf H., i try that

Uploading this file fixed it for me:

http://community.invisionpower.com/4docs/knowledgebase/407-sign-in-requires-two-attempts-r134/

​Fixed!