Like many of you, I too received this error. Here's my setup:
- WSS / sp2 / .NET 2.0 (appPool is using 2.0 as well)
- Son of SmartPart
To be sure here's my issue: I merged my controls with aspnet_merge (part of the Web Deployment project add-in currently in B2 that Scutt Guthrie's been talking up lately). NOTE: Without the capability of merging the half-dozen assemblies created when you "publish" a VS2005 website you will have to ensure that all of your child controls are loaded in various SoS parts - just so that you can load the parent control that calls them (SoS will not do it). Anyway, I moved my controls and assemblies (including Enterprise Library 2.0's data block - yeah it works inside of SoS) over to the UserControls & bin folders respectively. After it pretended not to load (fixed by recycling the browser) I noticed that certain postback events failed with the subject error.
I googled it numerous times... but nobody could actually fix it! I even asked a few SharePoint MVPs - all to no avail. Then I happenstanced across Jan's blog entry about ASP.NET 2.0 Web Resources and SharePoint... and it was like EUREKA - that's it! Thank you Jan! You are a lifesaver... but how about you update your blog with the fact that it also fixes THIS issue? At least then people who google the error will find the answer.
Update (3/22/2006): Jan has posted the link between "WebForm_PostBackOptions is undefined" and Son of SmartPart, and promises to update the installation instructions. Thanks again Jan - I'm positive this will help a lot of folks out.
Anyway I am now back on track... and the problem has completely gone away. I suspect that this should also be added to the installation instructions... since who isn't going to have a linkbutton or a validation control (what specifically broke me)? Also, with the capability to drop js's and images (since they don't load correctly sometimes due to a different directory structure on the server) why wouldn't we want this ASP.NET 2.0 specific magic bullet?!
Thanks again Jan (another SharePoint MVP - it seems like everyone is one these days) for making our lives just a little bit easier. You have now been officially added to my blogroll.