Two possibilities occur to me:

1. I think that Brothels have 3 upgrades in vanilla VI and now you have 4 upgrades. I assume that you did Brothel4's review panel icon and infopic.

2. An inherent problem of GnomeEditor: it adds a few lines at the bottom of the file when a Build, Unit, Projectile file is edited and saved with this tool. This lines aren't useful and sometimes cause errors. Open your building file with notepad and see if there are spare lines at the bottom and remove it (I would make a security copy of the file before).