How to find errors in a cable in the LAN?

The tester already measures the continuity.

If both ends have been properly mounted, which has already been repeated, the cable probably needs to be replaced.

I fear the error is more likely the termination. With open wires, that would already be ruled out. If only one wire were defective, it should still run at 100 MBit or some kind of connection should be established.

5 network cables with 8 cores result in 80 termination points. The probability that the (multiple) terminations are now the cause ... hmm. Twisted pairs can also be detected by a tester. I am more inclined to think of a defect in the cable. But you are right, if core 8 is broken, 100Mbit should work - cores 1, 2, 3, 6 are required for that.

But you are right, if wire 8 is broken, 100Mbit should work - wires 1, 2, 3, 6 are required for that.

Are you sure that a router decides on auto fallback to 100Mbit when a wire fails to conduct?

Normally, yes.

Perhaps the non-functioning socket is installed according to the B standard, the rest according to A, or vice versa?