Sorry for taking a while but I finally looked into your save file.
Interestingly, the global variable flag_taxi1_mission_launched ($397) is set to 1, which means the game thinks you're still on the taxi mission, thus preventing you to run another instance of it.
I've edited your save file to reset the variable to zero, so now the mission should be runnable
Check it please.
How the bug could ever happen?
This variable is set to 1 next to running the taxi mission, and reset to 0 in its cleanup routine - which is always called in both success/failure mission results. So, assuming this code is correct, I could only imagine one possible way of catching the bug: making a save while on a mission (using a CLEO mod for example), then leaving the game. In this case your save would have flag_taxi1_mission_launched set to 1. But saving while on a mission is a dramatically risky thing, as the mission script information will also be stored in the save file, resulting in almost 100% crash upon loading.
If you say you didn't save the game in this way - then the mission handler routine within the executable is possibly bugged.