Python's ForkingMixIn has a max_children setting, default 40. This can be too small for busy Roundup installations; roundup-server should have a configuration option to change this value, to avoid needing to patch the code along the lines of

--- roundup_server.py.ORIG      2022-10-02 14:38:23.607161827 +0100
+++ roundup_server.py   2023-04-17 10:29:40.848711578 +0100
@@ -770,6 +770,7 @@
         elif self["MULTIPROCESS"] == "fork":
             class ForkingServer(socketserver.ForkingMixIn,
                                 base_server):
+                max_children = 100
                 pass
             server_class = ForkingServer
         elif self["MULTIPROCESS"] == "thread":

to use a larger value.