I just found your question and with some additional information I found a really easy solution so I thought I'd share that too.
The problem seems to be, that there needs to be an Etag set for the skript to perform correctly. This Etag is
array('If-Match'=> '*')
To add this as a header to every UpdateCell request go into the source file in the library path:
library/Zend/Gdata/Spreadsheets.php
and change the UpdateCell method definition from
$entry = $this->getCellEntry($query);
$entry->setCell(new Zend_Gdata_Spreadsheets_Extension_Cell(null, $row, $col, $inputValue));
$response = $entry->save();
to
$entry = $this->getCellEntry($query);
$entry->setCell(new Zend_Gdata_Spreadsheets_Extension_Cell(null, $row, $col, $inputValue));
$response = $entry->save(null,null,array('If-Match'=> '*'));
Requests are now working for me as previously.