chaps

Hi, First i'm french, and my english is really bad, so, good luck to understand me. I have a big problem. I'm playing with a friend (by pbem), i have an imac and my friend a PC. We have played the first battle (it's an operation), i have put my troops for the 2nd battle , my friend receive the file, open it and... BUG.... he can do nothing, the cursor move but nothing works, he must switch off his computer. I have made a new file, same bug, and other files works !!! what happened. HELP Thanks for answer