L293D and PWM on enable pin: TTL voltage drops to 4 Volts
Well, my fault! I made a stupid mistake and forgot to wire the logic supply Vcc1. I didn't notice it because the pin wasn't unconnected (decoupling capacitor) and the motor did spin anyway (that's unexpected!). I did measure a current draw of 20-30 mA that caused the voltage drop and double checked the circuit. With Vcc1 in place there's no current at all.

So... that's it. Should I delete this question?