LR Error "attempt to index a nil value"

Question

As of this evening, Lightroom has stopped working. At various times including startup (2x) I get a message saying "?:0: attempt to index a nil value". Then when the program is loaded, it appears to have an empty catalog.
At first I thought the problem might be a corrupted catalog file. However, if I hold down ALT at launch and specify another catalog, or if I create a new one, I get the same results.
Within the program, I see the message again when I switch modules to DEVELOP, or SLIDESHOW but not when I switch to WEB or PRINT. Once I've moved away from the LIBRARY module, any attempt to move back to it results in the message and staying on the same module I was on.
I am running Windows XP Pro SP2. There is 2gb RAM on the system and 24gb free disk space on C.
I tried UNINSTALLING the program then re-installing with the 2.1 update, but that made no difference.
Something that may be related: I installed the MS .NET 3.5 framework in between the last time I used LR successfully and the first time I saw it fail. I don't know if there is a conflict there... everything else I've tried seems fine and I've got a BUNCH of software installed.

Answer 1

Jeannine,
     If you follow the advice of the previous poster Julie Kmoch and then gradualy add presets back in one at a time you might be able to find the one that is causing the issue. I may be guessing, but I believe the issue has something to do with either an incompatible Preset setting (eg a older preset in newer version of LR) or a corrupt Preset.
Say you bought LR2 and imported a bunch of Presets from the internet, cuz 'Hey these effects are cool and I don't have to recreate the wheel', and then you upgraded to LR3 (not a clean install) and now one of the Presets that calls a certain slider setting or value that is different in LR3 doesn't work anymore. This might be why it throws the error, but again I am just speculating.
Location of LR Preset folder in Windows Vista/ Windows 7
C:/Users/<YourUserName>/AppData/Roaming/Adobe/Lightroom/Develop Presets
If you do not see the folder AppData, you may need to change the settings in Windows Explorer to 'Show Hidden Files and Folders'. To do that in Windows Vista/ Windows 7, open a Windows Explorer window, Click on Organize, then Foler and Search Options. Then click on the View tab and select the option button next to Show hidden files, folders and drives.
Hope this helps a bit,
Glenn