The .NET framework shouldn't be a factor. I suspected your catalog at first, too, but your note that multiple catalogs exhibit this makes me suspicious of something in your preferences.
What I'd suggest trying:
1. Launch LR. Go to Edit > Preferences > Presets tab > click "Show Lightroom Presets Folder"
2. In Win Explorer, rename the "Lightroom" presets folder to "Lightroom-x"
3. Quit and relaunch LR. It will ask you to manually find your catalog.
4. See if the problem went away.
If that fixes the problem, I'd appreciate getting a copy of your problematic presets folder (most likely the prefs file) at jkmoch at adobe dot com.
If it doesn't fix the problem, you'll probably want to get rid of the new Lightroom presets folder that was recreated when you launched at step 3, and restore the old -x one. Then we'll have to look at your catalogs.
Had the same thing happen to me, but I thought it had something to do with the size of my catalog. I was creating a new catalog with LR3B and it was creating 1:1 previews when it hung at 34%. I have 340GB+ pictures that I wanted in one catalog (don't know if that is best practices or not, just what I have been doing). I had to kill the program (end task) and when I reopened it I got this same error as the previous gentleman: "?:0:attempt to index a nill value"
I did what you recommended above and it fixed the issue, but upon restarting LR3B it didn't ask me for the location of the catalog. All my pictures are there, but my Develop Presets and possibly other things are not, oh well.
If you want my Prefs folder let me know.
While uploading images to smugmug for a wedding I just did I got this message. It really freaked me out so I just started backing up all my stuff. Here is my concern, is it in one of my presets that I used to edit my images? Is there a way to figure out which preset it is. I have only 347 images left and I could look for the presets left that are on those photos. Maybe I could then just delete that preset instead of all my wonderful presets? I haven't had any issues but an error message when my computer is trying to finish the last 347 images of 1580 already uploaded. LR still seems to be working just fine. So I went in and exported all the images to a folder on my desk top because I don't have a current back-up....lessoned learned and cross your fingers for me.
Any explanations or advise would be so amazing...thank you ahead of time.
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
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,