So I had an error message pop up while trying to enter shows saying that the barn manager was already entering. I don't have the barn manager hired - I've never used her. At the moment I can't enter shows because of this.

This happens to everyone. At roughly 4:20pm game time, the script for the BM runs. It's to prevent people from entering shows while the BM is dumping thousands of horses into shows.

Just wait a few moments and she'll be done.

Like Eury said, it isn't a bug. The script runs for about 20 minutes every day, whether you use the barn manager or not.