I download the WoW PTR client to my WoW folder, with the "Repair", "launcher" etc. When I click on the PTR installer with the shooting stars another window pops up, I click continue. A window pops up telling me "Sorry, the installer was unable to start up." and below that it says. "The installer requires "Wow.exe" version 3.0.1.8874 or later." i already looked at the Wowwiki guide. Didn't help
I download the WoW PTR client to my WoW folder, with the "Repair", "launcher" etc. When I click on the PTR installer with the shooting stars another window pops up, I click continue. A window pops up telling me "Sorry, the installer was unable to start up." and below that it says. "The installer requires "Wow.exe" version 3.0.1.8874 or later." i already looked at the Wowwiki guide. Didn't help. Yes I have the newest version of WoW. Was just on retail before maintenance today.

Are you fully updated to the newest version of retail WoW?