When selecting a style, or generating a song, I get various errors such as "Cannot open file bbinput.txt" or "cannot access the file because it is being used by another process".

RealTime features in some anti-virus programs can interfere with the plugin when it needs to communicate with Band-in-a-Box for Style selection and generation. This can result in the error messages mentioned, or cause a long delay, for example when opening the StylePicker. You should make sure you have downloaded the latest update patch for your version of Band-in-a-Box, which will also install the latest DAW Plugin update.

We've noticed this most frequently with Malwarebytes, and you could prevent the potential delays by adding your bb folder to it's Allow List. Other AV programs have a similar "allow" or "exclude" list.