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I had this problem once with a filmback that came with a lot of used Hasselblad gear.
Good chance your back has a similar problem.
If you have another back to check how the insert looks like it will be easier to understand what I will try to explain.
Take the insert out of the good back and look at the plate that presses the film to the guides.
At the side opposite the locking knob you see a metal rim that locks the film when you place the insert into the housing.
With your blocked filmback this rim is probably damaged and prevents the insert coming out.
Remove the darkslide and try to use a thin object like feeler gauges to press the rim down to clear the insert.
You may use force as the rim is already damaged and needs to be replaced anyway.I hope this helps.
Do not forget to write to the forum that you solved the problem.
I think that is a sensible decision.
I was under the impression that it was all up to you to solve the problem.
If you need a good repair man I may be able to recommend one depending on where you are of course.
You can send me a pm through the forum.