Hi,
I had a similar problem with a new opened franchise zoo. The messages came while a mechanic was working on the power supply, so it was placed reachable.
A few minutes later I saw many visitors on the main path literally running against an invisible wall. After removing a part of the path and then reconnecting it, the visitors could go to the habitats and the message for the power supply was gone.
The power supply is on the other end of the zoo, with a nearby staff area (but not organized in work zones at the time). So I think the problem was another technician who couldn't reach the supply.
Maybe there is a similar problem with a path somewhere in your zoo