Thanks for the advice. I have started by contacting IB on a forum (cfr. thread on Elitetrader.com) and I will later issue a ticket. However, I have also found that it is even more useful to ask other users.
Due to other reasons, I finally resolved to update my customized twsdde.xls from the version I used years ago to the latest version, which dates to Fall 2012.
It took me about 2 days of work.
At the end, I found out that the "SIU3" method (cfr. above posts) is still needed for historical data requests (on the "historical data" sheet), because the "multiplier 5000" method only works on the tickers and the orders page of the latest twsdde.xls.
On the previous (customized) version of twsdde.xls, instead, I had to use the "SIU3" method on every single request: orders, tickers, historical data.
Finally, I should mention that twsdde.xls hasn't changed much in the last few years. The only page with important changes is the tickers page, which has 4 extra columns. Other than that, just a few minor cosmetic changes, and then there are some changes in the VBA modules.
But I had to update it because my orders sheet wasn't sending orders anymore. Now everything works again.