Drag and Drop Mail attachment won't work stable since 10.9

Question

Hi,
this problem occurred on my system after upgrading from 10.8 to 10.9 and is still present under 10.10.2 (besides the well known WIFI issues that weren't cured at all with 10.10.1 nor 10.10.2)
After some time of work, I'm not able to copy attachments from mails into new mails or in finder by simply dragging. The mouse cursor won't grab the files any more as if this feature was never present.Restarting Mail.app will help for some time but the error reappears.
I've checked system.log and was able to identify the following log as soon as the bug appears.
Feb  4 11:29:06 MBA11-2.local Mail[18679]: *** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[0]
Feb  4 11:29:06 MBA11-2.local Mail[18679]: (
  0   CoreFoundation                      0x00007fff8a10b66c __exceptionPreprocess + 172
  1   libobjc.A.dylib                     0x00007fff906ac76e objc_exception_throw + 43
  2   CoreFoundation                      0x00007fff89ff8a4f -[__NSPlaceholderDictionary initWithObjects:forKeys:count:] + 383
  3   CoreFoundation                      0x00007fff8a03f4eb +[NSDictionary dictionaryWithObject:forKey:] + 59
  4   AppKit                              0x00007fff94113619 -[NSCoreDragManager beginDraggingSessionWithItems:fromWindow:withClipRect:event:source:] + 1263
  5   AppKit                              0x00007fff94112f94 -[NSView(NSDrag) beginDraggingSessionWithItems:event:source:] + 295
  6   MailUI                              0x00007fff915d1cad -[MUIWKView mouseDragged:] + 1652
  7   AppKit                              0x00007fff94591a02 -[NSWindow _reallySendEvent:] + 4507
  8   AppKit                              0x00007fff9401a16e -[NSWindow sendEvent:] + 446
  9   AppKit                              0x00007fff93fcc451 -[NSApplication sendEvent:] + 4183
  10  Mail                                0x000000010108f940 Mail + 272704
  11  AppKit                              0x00007fff93e58608 -[NSApplication run] + 711
  12  AppKit                              0x00007fff93e43a14 NSApplicationMain + 1832
  13  libdyld.dylib                       0x00007fff92ecf5c9 start + 1
Any thoughts?

Answer 1

Quit Mail. Force quit if necessary.
Back up all data. That means you know you can restore the Mail database, no matter what happens.
Triple-click anywhere in the line below on this page to select it:
~/Library/Mail/V2/MailData
Copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder
from the menu bar. Paste into the box that opens by pressing command-V, then press return.
A folder window will open. Inside it there should be files with names as follows:
Envelope Index
ExternalUpdates.storedata
Move those files to the Desktop, leaving the window open. Other files in the folder may have longer names that begin as above. Move those files, if any, to the Trash.
Relaunch Mail. It should prompt you to re-import your messages. You may get a warning that the index is damaged and that Mail has to quit. Click OK. Typically, the process takes a few minutes, but it may take hours if you have gigantic mailboxes. In that case, you may be able to speed things up by temporarily adding your home folder to the Privacy list in the Spotlight preference pane. Remove it when Mail has finished importing.
Test. If Mail now works as expected, you can delete the files you moved to the Desktop. Otherwise, post your results.