Time machine never ends backing-up (and the backup disk is NOT faulty)

Hello,

I've been wasting a lot of time with time machine lately. As soon as it reaches something like 80% of the back up, the ceiling (volume of datas to be saved) increases continuously as fast as the volume of datas eventually saved. So that it leads to an infinite process, and the backup disk storage limits are obviously reached anyway ...

I've tried many things already : I supected the time capsule unit but it also happens the same way with ANY external hd. I excluded many folders from the lTM pref list, but nothing helped so far.

I do have the very same system (Mojave) on my MacBookPro and time machine works perfectly on it.

But I don't have the same files/apps on my MBP than on my MP, so I suspect some files might corrupt the process,.

Any help would be greatly appreciated

Thanks very much in advance