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When the door is open and you fiddle with the inside handle, do you hear anything or feel any resistance? Nothing moves in the latch area? You will probably have to remove the door panel to get a look inside, it might be something quite obvious in there that you will see right away. Take the panel off and post pictures.
Go to YouTube and type in: C3 Door Handle Repair.
Then you can get a visual of what's going on inside the door. Take note of the small parts, clips, clamps, etc on the door rods. Those small parts can be found on EBay for a song, free shpg.
What he said, but you might play with the latch post on the door sill. You can loosen it with an allen wrench and move it just a bit in or out, tighten it, and see if helps.
I'm thinking that if it opens fine from the outside. Fiddling with the latch is truly a waste of time. Perhaps one of the clips on the rod popped off and the inside lever is just disconnected. You need to get that door panel off.