I did some further investigation and I think it has to do with AD users in osTicket.
If I create users, which are not in AD and don’t share the same first given name as anyone else in AD (search does work):- John Smith- Jen Johnson- David Miller
If I create users, which are not in AD but have the same first given name as someone else in AD search does not work:- Andreas Lopez- Franz Clark- Markus Moore
*If I start searching for their first given name + surname it works.
And as before search for AD users does not work- Andreas ***- Franz ***- Markus ****Works only if I cut the first letter of the given name, by adding a . at the end of the given name or by surname