Have you contacted the guys at 360Works? They will probably have the best information about what is causing this error.
Yes, I posted a similar message on their support forum. Still awaiting a response.
The reason I posted here, was that I cannot find any documentation from FileMaker on error 474. I've checked the posted documentation and technical support database. Nothing.
I was hoping to get some feedback suggesting things to try. Unfortunately, the error is not specific at all. It would be nice if the error gave a more specific reason. Such as "incorrect permissions on file" or "not a valid plugin file" or "failed checksum" or whatever.
I've encountered this error before, and usually it was when the permissions were not set correctly.
I can't explain why, but I've encountered instances where changing the ownership via Finder's Get Info wouldn't work (even though it appeared to do so), but using chown in the Terminal finally did. I'm confident that's not the case this time, but last time that happened I got the same "Error 474" over and over until I happened on the solution...
I've verified the permissions so I'm not sure what else to try. Others appear to be using this updated plugin fine, but I have no direct evidence.
I will try it on another server when I get a chance.