I had an issue with Ageas Retail where every time they were registering users on only 2 servers - that they were getting failed to register user error.
We tried everything and worked fine if we moved them to another server.
The issue was a database replication issue.
Because they operated a multi-server cluster. This means that they had 4 slave servers and a master.
When we looked in SQL studio manager at the publications; where server replications from SafeCom are kept there was only 2 servers, not the other 2 having the issue.
We feel it must have been removed manually by mistake.
But SQL team advised they have never removed any.
I had to give the SafeCom account sysadmin rights again; it does not require these rights to run normally but if you need to repair replication you will need this rights.
Once these rights were given.
I then went onto SafeCom administrator, right-clicked each server and repaired replication.
Once it repaired successfully - they all appeared in publications within SQL.
FYI, you cannot repair replication of master as this is where they are all pointing; ignore master one.
Tested and now users on all servers can register fine.