Nextbike have some very shady business practices. Don't ever have a negative balance with them. They won't tell you and will proceed to threaten you with legal action a year later. Happened to me over £3 (which later became £10 as they don't accept payments below £10).

They also don't comply with GDPR. After confirming several times they have completely deleted my account and any reference to my email, I still get marketing emails from them every now and again.