I got that error when the pathname to the wallet.dat file contained a space. I put the file in "C:\temp\" and then it worked.
I also found that I could only successfully import wallet.dat files that are *older* than mid-May.
I've seen quite a few reports of errors opening the wallet file when importing, but this is the first time I've seen it explained.
I wonder if this is only occurring on Windows.
The initial CLAM distribution was made to all BTC, LTC, and DOGE addresses which held a non-dust balance on May 12th 2014, which explains your mid-May observation.