This is caused by a conflict on the Fund Association tab of the Fund. If an advisor is listed twice on the fund association tab with different rep type codes, but one has Yes for accessing Donor Central and the other No, this can cause a conflict and can cause donor user names to randomly disappear from Donor Central.
To resolve this issue, all rep types for the same advisor need to be set for Yes to be able to access Donor Central.
This can also be caused if the System Generated UserID that was created when the advisor was setup in FIMS was changed. When the advisor is setup in FIMS, the system generates an ID and that ID gets sent to FIMS. If the user id is changed that change may be delayed in reaching Donor Central. Typically in that case a manual sync to Donor Central will resolve the issue.