On the page you quote, "Error on every startup...", post 7 from Geoff the Kiwi references an Adobe document which explicitly details the location of things like the Preference File; this surely tells you where to look for them?
The preference file is irrelevant to this error. Leave it alone.
To delete the preview cache, do this:
1. Quit Lightroom.
2. In your Operating System, locate your Lightroom catalog. Typically catalogs are in /Pictures/Lightroom on Mac and My Pictures\Lightroom on Windows.
3. Find the folder whose name ends with "Previews.lrdata" and delete it. (note, you may also have a folder whose name ends with "Smart Previews.lrdata." You shouldn't need to delete that one.)
Actually, I didn't specifically suggest the OP should touch the Preference file; the Adobe document I mentioned lists the locations of all relevant files, including the preview cache which the OP was looking for. In my post I said it " details the location of things like the Preference File ".
The preview cache is in a completely different folder location to "things like the preference file". The OP needs to be told to stop looking for preference files, and how to resolve this problem.
Yes, John, the Previews.Irdata is indeed in a totally different location to the preferences file, but to repeat myself again, I did not specifically point the OP at it but told him where he could find a list of the files installed by Lr when it's installed on either a Windows or Mac.
I'd be interested to be shown where in my posts I specifically told the OP he should alter/ delete the Preferences file.
Anyway, how have you got on, Michael W?
sorry for the late reply.. we had some electricity issues so i couldn't really test it..
but your answer helped me to solve the problem.
first i also deleted the Smart Preview after that i was able to enter the catalog but there was no preview of the pictures anymore..
luckley i had a backup so i put them back in the folder after i saw your post.
so thnx a lot!