Voltage regulator is built in with the alternator. You could have the alternator rebuilt at a starter alternator re-builder, or you can just buy a re manufacture alternator.

Does this sound like a voltage regulator problem?

If the voltage at the battery is over 12 volts, but the voltage at the starter or alternator or jump terminal is below 12 volts, then you have a voltage drop somewhere in the electrical system. Could be as simple as a bad ground.

I suspect the problem is a bad or clogged transmission solenoid. Since the solenoid is inside the transmission, accessible by removing the pan, I would eliminate other potential causes first.

If you don't know when the codes were last cleared I would clear them and see what comes back first.

Next, I suggest you unlock the cluster and monitor the charging system. A bad battery or one that has a low charge will cause all kinds of electrical gremlins. Eliminate the charging system as a potential cause.

I would expect low transmission fluid to cause more issues than you have mentioned but it is easy to check. Drive enough to get the fluid near operating temp. Turn off the engine and pull the fill plug. You should see a small amount of fluid come out the fill hole or the fluid will be at the fill hole level. If it doesn't add proper fluid until it does. It will have to be down a considerable amount to be the cause. The vehicle needs to be level for this process.

At that point it is time to see a transmission shop that is familiar with BMW transmission for a diagnosis. You could get a diagnosis first and have them check fluid level when they do so though diagnosis may cost $$$. Dealers don't have transmission expertise.