Can't open session because of unknown source

I am again locked out of a session because of some missing source (midi). I have noticed now that it happens each time I cleaned the unused sources and regions. Can someone explain to me how I am supposed to use the cleaning tool? Or should I not use it? And what should I do to fix the xml (i.e. deleting the line) without loosing the take I recorded?
I used Ardour for many years now (not that I am an expert, far from that) but I never experienced such behavior before. Help would really be appreciated. Tx

Just an assumption… May be the reason of disabling to open the session - plugins. Did you try to open with checkbox activated: “Safe Mode: Disable all Plugins?”

Which version of Ardour are you using?

Thanks for answering. Yes, I’ve tried that, no effect. I use few plugins anyway, only some midi filters and the like.

The last version: Ardour 7.1.

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