After upgrading to v1.9.4-RC1 I found that I couldn't login to my forum and couldn't send myself a reminder as my user name was not found.
The solution for me was in the membership provider section of web.config. The v191-upgrade-web.config file recommended:
<membership defaultProvider="YafMembershipProvider" hashAlgorithmType="MD5">
<providers>
<clear/>
<add connectionStringName="yafnet" applicationName="YetAnotherForum" name="YafMembershipProvider" requiresUniqueEmail="false" type="YAF.Providers.Membership.YafMembershipProvider" hashHex="true" hashCase="upper" />
</providers>
</membership>
With this section I could not login, so I changed it to
<membership defaultProvider="YafMembershipProvider" hashAlgorithmType="SHA1">
<providers>
<clear/>
<add connectionStringName="yafnet" applicationName="YetAnotherForum" name="YafMembershipProvider"
requiresUniqueEmail="false" type="YAF.Providers.Membership.YafMembershipProvider" useSalt="true" />
</providers>
</membership>
Now I can login again and have also tested the sending of password reminders, which works too.
Possibly my problem was that I didn't check which version I was running before starting the upgrade. I could well have been running something later than v1.9.1 and caused the problem that way.
YMMV.