Titler Pro 4 and 5 host third-party OpenFX plugins. Sometimes there can be conflicts that prevent Titler Pro from opening successfully.
On Windows, try renaming or deleting OFXBridgeB64.dll from C:\Program Files\NewBlueFX\Common, then try opening Titler Pro again.
On Mac, delete OFXBridgeB.bundle from Macintosh HD\Library\Application Support\NewBlue\Common, then try opening Titler Pro again.
If the problem persists, please contact support.