As I understand it, may be incorrect, the best way of sorting matches out is to have a mother, father and child all tested. You can then see what has been inherited from each parent, and child can assign them as paternal and maternal on each chromosome.
Without this, if you match someone in the same place on the same chromosome, you cannot be sure whether it has been inherited from the father or the mother, so a promising match could prove false.
Again, as I understand it, open to correction, if you have fairly close matches and can identify MRCA, you can use those matches to triangulate that line.
That is, my paternal first cousin twice removed can be used to help triangulate a new match who matches in the same place on the same chromosome. The new match could still be a false match, or a maternal match, but the more you find, the stronger the connectIon becomes, but you preferably still need some sort of paper trail to confirm. Adoptees sometimes have no paper trail whatsoever, but can still find out about their probable parentage.
However, there are common pile up areas, where lots of people will match you, but perhaps from a long time ago.
I have had relatively few matches outside ancestry, so am hoping to learn more about this, but it seems a slow process (the learning that is).
Keep on trying with myheritageDNA, haven't had any problems uploading there myself, only at ftDNA, where you need to use a third party tool.
Regards Margaret