In Croatia, Sajdner families are Croats (from Zagreb ).
Prevalence
Only a few people with faimily name Sajdner live in Croatia today, in one household only. There were 0 of them in the middle of the past century, and their number remains constant. They are located in one Croatian city only, in Zagreb (<5).
Outside Croatia
Family name Sajdner is present in 22 countries worldwide. "Seidner " is used by some 1000 people in the United States , about 300 in Germany , and some 100 people in France.
In the sources analysed in the project Acta Croatica so far, the name is mentioned few times, the first time 2007 in source Encyclopaedia of Croatian Surnames.