If there is a setting for the calendar owner to enable or disable outgoing feeds, check (or ask the owner to check) if this setting has been changed to disabled.
Check if the feed URL has the correct permissions. For example, a public Google Calendar feed only works if the calendar itself has been made public.
If the feed URL is valid and settings are appropriate:
Reset the feed in your Teamup calendar.
When a feed import fails 10 times in a row, Teamup stops fetching the feed. To reset the feed, go to Settings > Calendars, open the affected feed, and save again. This restarts feed synchronization.
Details on the source of the feed: Which calendar service does it come from? Is it a business or a personal calendar? Who maintains the calendar? Do you have access to the source calendar to make adjustments or to obtain a new feed? Was the feed working prior to this issue? When did the feed stop working?
The error code
When a feed fails to import, the feed configuration form usually shows the last error message. It is very helpful for us to see that error message.