Gears

You don't mention any grinding noises so I'm assuming it's not an internal gearbox fault.
So I'd suggest it's a fault with the linkages,
there's 3 that translate the gear lever movement and they're prone to wear.
A little awkward to fit but no real dismantling.
they're double ended ball/socket beasties.
they just lever off and then press/pop the new ones back on.
Around £30 for a set from Ebay,
Pretty common fault,
google berlingo gear linkages,
there's even a youtube video instruction.

Have you had a look at this similar problem:

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You could try lubricating the gear linkage to loosen it up.

You could try lubricating the gear linkage to loosen it up.