Just thinking out loud:

What router/switch (brand / model) do you have on the new building, where the network is being originated? What brand/model do you have in the old building?

Have you then tried to force 100Mbps on all the ports involved (switches of the old and of the new building)?

What about redoing terminations for all cable ends?
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= Taym =
MK2a #040103216 * 100Gb *All/Colors* Radio * 3.0a11 * Hijack = taympeg