The problem is that the current release version of Experimental is based on what is now an old version of Standard. Newer versions of Standard have a different version of the save game file, so these cannot be loaded by current versions of Experimental. Bernd Gabriel has been doing a great deal of work on merging the latest version of Standard into Experimental, so this should be fixed in the next major release.
In the meantime, however, there is a workaround to this. If you change the saved game version of the game that you are saving in Standard from 112.6 to 112.3, the game should be readable by the current release version of Experimental, although some data (certain graphs, for example) will not be saved. To change this setting, you need to access the advanced settings dialogue box. In Pak128.Britain-Ex, this is accessed by pressing the "i" key: I cannot remember how this is accessed in other paksets, but this can easily be checked (if necessary by posting in the "Help Centre" forum.
