netrs
Are you referring to the Pseudonyms panel in server manager ?
Where & how do you create an alias ?
Please read the Pseudonyms section of the Manual, it explains how Pseudonyms work with multiple domains & limitations that apply.
You say "It was working fine for many years and then suddenly stoped".
So what changes did you make to the server ?
Please give full example of what you do, & what you expect to happen, & advise us what actually happens, step by step please.
My next step I guess is to make a new clean install and restore the last backup.
That might fix the problem but it also might NOT fix the problem. At this stage the problem has not been identified so it is guessing.
You need to tell us the history of the server & what changes you have made etc.