Exception “Invalid cell coordinate V”

Question

I'm trying to read an Excel2007 template, edit it and save it to a new Excel2007 file.

The template is made of 7 worksheets, with 50 cols and 2000 rows per worksheet, and it weights 2.6Mb empty.

When I attempt to save the file, even if I do nothing with the template, I'v always got this exception :

Invalid cell coordinate V

I understand that somewhere in the template, a formulae is looking to a cell with its column, but no row is specified. I can't found any formulae like this.

I'v noticed if I specify

$PHPExcel->disconnectWorksheets();

before saving, the exception is not thrown, but saved file is corrupted.

--- EDIT ---

I forgot to mention that my template have many combo box. Is that what could cause this error ?