ABS and brake warning light, error codes 00290 00688

Hello!

Volkswagen Golf III AFN, manufactured in 1997, model year 1998, with 147,000 km (approximately 91,000 miles).

"For the past few days, I've been seeing the ABS warning light and getting this error code:"
00290 (Rear left speed sensor) 35-00.
Initially intermittent, now constant.
Okay, it looks like I'll have to replace (or have replaced) a sensor again, but today, in addition to the ABS light, something else happened.
Sometimes, the brake warning light comes on for a few seconds when starting the engine.
and subsequently, an additional error code appeared:
00668 (Supply Voltage Terminal 30, 16-10 Signal - Outside Specifications)
Okay: On-board voltage at terminal 30 is outside the tolerance range.
The error database indicates "Interruption, short circuit to positive or ground."

Battery voltage is good (12.5V after a long period of inactivity), and the relays seem to be at a normal temperature.

What is that likely to be?

Thank you for your responses.

Greetings, Klaus.

/viewtopic.php?t=7901