Cannot set userid temporarily unavailable

When trying to su to a specific user, I get the error "su: cannot set user id: Resource temporarily unavailable".

All the research I have done so far indicates the problem should be in the /etc/security/limits.conf file. However, even after adding a specific line for this particular user, it still throws the error.

ps -eLF | grep ^user | wc -l
returns with 669.

My personal account that has sudo privs can't assume that user, nor can root. I can assume other users.

At a loss. Ideas?