I think this bug has been fixed in a more recent version of FreeMind, but you may go through this with version 1.0.1 for Linux, possibly for Windows also:
1. Try to open a map
2. Get this error
This is clearly a bug, and I haven't seen it with newer versions of FreeMind on Windows. A workaround is to fix the first line of the .mm file in question:
<!-- To view this file, download free mind mapping software FreeMind from http://freemind.sourceforge.net -->
<node CREATED="1386726098959" ID="ID_368233367" MODIFIED="1386726102812" TEXT="git">
To resolve the issue, replace "0.9.0" with "1.0.1" using a text editor, save the file, and you should be able to open the file again.
If you want to update a large number of maps try the following (Linux command line):
sed -i -e 's/0\.9\.0/1.0.1/' *.mm
The above command will look for "0.9.0" and replace it with "1.0.1" in all files named *.mm, and the -i option means it will update the file in place.
(You can also use sed under Windows, see the unixutils package at Sourceforge)