If Trace All Feeders is interrupted, any attempt to run Trace All Feeders or Trace A Feeder will throw an error to the user like the following. If running Trace A Feeder in either ArcMap or ArcCatalog, the message persists even when the user chooses to proceed.
When Trace All Feeders is run, a flag is set for that geometric network's entry in MM_SYSTEM_PERSIST_INFO. This flag is cleared when the process completes successfully. This flag prevents two users from running Trace All Feeders at the same time, which as a process that works on every feature of the geometric network in every version is not something that should be run simultaneously.
The most direct solution is to simply run Trace All Feeders again and allow it to run to completion. This will clear the flag and remove the warning message.
If Trace All Feeders cannot be run again due to data issues or time constraints, the geometric network's entry in MM_SYSTEM_PERSIST_INFO can be deleted and recreated. This will of course delete any changes to Feeder Manager System Settings on the geometric network, which will need to be reset if needed afterwards by right clicking on the geometric network and choosing Feeder Manager > Settings. Once the row is deleted the flag will be removed and users will no longer receive the warning message.