temporary hickups: "LDAP account "<snip>" does not exist anymore, blocking Gitlab user "

We're faces with a problem of a heavily used technical user, that GitLab spews a log line like this:

September 28, 2018 14:46: LDAP account "..." does not exist anymore, blocking Gitlab user "..." (...)

which we can only circumvent by login with that account into GitLab, which yields into

September 28, 2018 14:51: LDAP account "..." is not disabled anymore, unblocking Gitlab user "..." (...)

Is this a bug in Gitlab 10.6.4? How can we stop GitLab from blocking a technical LDAP user?