We’ve seen several reports that Logic Pro 9.1.7 is having difficulty validating 3rd party Audio Unit plugins since the Lion 10.7.5 update. This appears to happen with most 3rd party plugins. This includes powered plugins like UAD.
We therefore recommend not updating to Mac OS 10.7.5 at this time if you are a Logic user. Watch for updates and potential fixes.
Here are some discussions regarding the topic:
UPDATE: Apple released a supplementary update that fixes some third party validations. The link is below.