Blackberry - No Synch "Can't open default folder in group"

on the Outlook side, from Help, do a Detect and Repair.  That should correct the PST store. (sound like you've done this already, but in case I'm mis-intepreting...)

The "can't open default folder in group." sounds like it is from the BB side; considering that you are now working with a new PST file.  Sadly, there is no BB side repair tool (at least mainstream anyway).  So, just to be safe, you may want to do a backup of the BB and note all of the new items found on BB and not on Outlook (just in case you have to input contacts, etc, manually).  Wipe out the BB, power down the BB, take the battery out for 30 seconds and then either restore, or if the corruption got backed up; do the wipe out process again and just sync to your outlook.

Regards,

Here is the support article from the BlackBerry website.  This is a common problem:

http://www.blackberry.com/knowledgecenterpublic/livelink.exe/fetch/2000/8021/7919/7921/8123/Support_-_Unable_to_open_default_folder_in_group.html?nodeid=702448&vernum=0

Problem
When synchronizing the personal information management (PIM) applications, you receive the following error:

Unable to open default folder in group


Cause
Intellisync® is unable to communicate with the selected Microsoft Exchange folder.

Resolution
Clear and restore the translator for the application that was synchronizing when the error occurred.

1. Connect the handheld to the personal computer and open Desktop Manager.
2. Double-click Intellisync and click Configure PIM.
3. Select the application and click Choose.
4. In the Choose Translator selection window, select your email client. Under Folder Name, click Browse.
5. In the Selected Folders section, highlight the entry and click Remove.
6. Expand your mailbox and add the folder you just removed.
7. Click OK to close the window and return to the main Intellisync window.
8. Click Synchronize now.

Workaround
Back up your handheld data. Clear and restore the handheld database for the application that was synchronizing when the error occurred.

1. Connect the handheld to the personal computer and open Desktop Manager.
2. Double-click Backup and Restore and click Advanced.
3. In the right pane, select the database. Click the arrow to transfer the database to the left pane.
4. Click File > Save As and save the database on the personal computer.
5. Select the database in the right pane and click Clear. In the warning dialog, click OK.
6. Click File > Open. Find the backup file you saved in step four and click Open.
7. Select the database in the left pane. Click the arrow to transfer the database to the right pane. In the warning dialog, click OK.
8. Open Intellisync again and click Synchronize.

Neither of the above answers resolved the issue. What did resolve it was the following using the BB Knowledge base at www.blackberry.com:

This "correct" answer was provided to me by someone from the Blackberryforums.com:

KB # 04427 To look at the log file you will need to open the tif.log located back in the Intellisync folder. It will open with Notepad, so just scroll right to the bottom of the document and you will then notice a value in there called subject= "name of subject" (what you need to look for is the subject name in the quote marks)

Then you need to do an advanced find in your calendar in Outlook and remove that entry in the quote marks. Then you need to do another sync, if you don't get any more errors then change the values back to normal in the ilopt.cfg file.