We got frequently this exception:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/roundup/cgi/client.py", line 719, in inner_main
    self.write_html(self.renderContext())
  File "/usr/lib/python3.8/site-packages/roundup/cgi/client.py", line 1866, in renderContext
    tplname = self.selectTemplate(self.classname, self.template)
  File "/usr/lib/python3.8/site-packages/roundup/cgi/client.py", line 1859, in selectTemplate
    raise templating.NoTemplate('No template file exists for templating '
roundup.cgi.templating.NoTemplate: No template file exists for templating "issue" with template "calendar" (neither "issue.calendar" nor "_generic.calendar")


I think it is due to crawling bot but they keep trying because roundup return a status 500.
I think the NoTemplate exception should be catch in renderContext also when calling selectTemplate, here is a patch that does that.