version 8.4 2018-07-26 (build 84192014), Windows server and Windows clients.

I have a maintenance which generates PDF files from Invoices on the server. The number of PDF files generated is over 3000.
All users get the message "server busy" while the maintenance is running.

Why this happens?
I expected the maintenance to slow down a bit the performance, but it actually blocked all the users (about 10)
my code:

global
procedure GenInvPDFTrMn(record RcVc RepSpec)
begin
....
while (qupdating.GenInvPDFTrMn_Run(RepSpec,path,true,prevlooppos)) begin
prevlooppos=RepSpec.long1;
end;
LEnd:;
SetFileOnServer(false);
return;
end;