How to find errors in a cable in the LAN?

Maybe the non-functioning jack is wired according to the B standard, the rest according to A or vice versa?

That would concern the entire pair, so in the case of 8 also 7.

Are you sure that a router opts for auto fallback to 100MBit if one wire doesn't conduct?

Usually yes, unless a specific transmission mode has been fixed.

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

But the tester also shows that. Cross-pair swaps are nowadays corrected automatically (no more need for crossover cables ;-))

Normally yes, unless a specific transmission mode has been fixed



But the tester also shows that. Crossed pairs are nowadays also automatically corrected (no need for crossover cables anymore ;-))

Yes, that should be the case. Although I wondered what happens if only a CAT cable is initially connected to the device at such a socket... the outlet cannot negotiate.

Measure the conductor against the shield. You probably have an insulation fault (if continuity).

That would affect the entire pair, so with 8 also 7.

Conductor pair 7/8 (brown/white + brown) are the same in TIA 568 A and TIA 568 B, so it doesn't matter here. My tip: borrow a Fluke DSP or similar, measure and locate the fault to within a few centimeters. I could help if you are nearby? (NRW / Münsterland)