I started a new game in 2.6-dev and the first mission I tried encountered the error in the title. Here is some more output from the log:

Load 'save/campaign/slot1.savx' 21 subsystems
...Running subsystem 'campaign'
UI_DeleteNode: Can't delete the static node 'geoscape.messages.dock.bottomMargin'
...subsystem 'campaign' - loaded.
...Running subsystem 'research'
...subsystem 'research' - loaded.
...Running subsystem 'base'
...subsystem 'base' - loaded.
...Running subsystem 'hospital'
...subsystem 'hospital' - loaded.
...Running subsystem 'market'
...subsystem 'market' - loaded.
...Running subsystem 'employee'
...subsystem 'employee' - loaded.
...Running subsystem 'aliencont'
...subsystem 'aliencont' - loaded.
...Running subsystem 'aircraft'
...subsystem 'aircraft' - loaded.
...Running subsystem 'alien base'
...subsystem 'alien base' - loaded.
...Running subsystem 'interest'
...subsystem 'interest' - loaded.
...Running subsystem 'installation'
...subsystem 'installation' - loaded.
...Running subsystem 'mission'
...subsystem 'mission' - loaded.
...Running subsystem 'ufostores'
...subsystem 'ufostores' - loaded.
...Running subsystem 'production'
...subsystem 'production' - loaded.
...Running subsystem 'messagesystem'
...subsystem 'messagesystem' - loaded.
...Running subsystem 'stats'
...subsystem 'stats' - loaded.
...Running subsystem 'nations'
...subsystem 'nations' - loaded.
...Running subsystem 'transfer'
...subsystem 'transfer' - loaded.
...Running subsystem 'xvirate'
...subsystem 'xvirate' - loaded.
...Running subsystem 'messageoptions'
...subsystem 'messageoptions' - loaded.
...Running subsystem 'triggerevents'
...subsystem 'triggerevents' - loaded.
UI_DeleteNode: Can't delete the static node 'geoscape.messages.dock.bottomMargin'
File 'save/campaign/slot1.savx' successfully loaded from compressed xml savegame.
Music: track changed from van_theme to van_geoscape.
Music: track changed from van_theme to karlmacklin_geoscape.
Calling subsystems
...subsystem 'campaign' - saved
...subsystem 'research' - saved
...subsystem 'base' - saved
...subsystem 'hospital' - saved
...subsystem 'market' - saved
...subsystem 'employee' - saved
...subsystem 'aircraft' - saved
...subsystem 'alien base' - saved
...subsystem 'interest' - saved
...subsystem 'installation' - saved
...subsystem 'mission' - saved
...subsystem 'ufostores' - saved
...subsystem 'production' - saved
...subsystem 'messagesystem' - saved
...subsystem 'stats' - saved
...subsystem 'nations' - saved
...subsystem 'transfer' - saved
...subsystem 'xvirate' - saved
...subsystem 'messageoptions' - saved
...subsystem 'triggerevents' - saved
XML Written to buffer (127435 Bytes)
setting game random seed to 78280
------- Loading game.so -------
not found at '/usr/local/lib/'
not found at '/home/mark/.ufoai/2.6-dev/base'
found at './base'
********************
ERROR: CM_AddMapTile: fighter_crash has wrong version number (79 should be 78)
********************
Shutdown server: Server crashed.