Intermittent error "ORACLE user account"

We have had a few instances of different users receiving the error message "Encountered an error while getting the ORACLE user account for your concurrent request".  This happens while performing a particular process on an employee, but only rarely - and they are able to perform the same process on other employees without any issue.  Also, other people can perform the process on the employee who failed and have it work.  I've searched KB for the error message, and the only reference is in connection with absence requests, which is not what is being done here.  Does anybody have any