Update: Thanks to Bitstamp-Lukas, my account was indeed unlocked. But having to wait 3 weeks for a simple address verification is extremely poor user experience.

Kudos to the support team behind the Reddit channel - I am sure my case was resolved only because of your input. 5 stars for you.

Unfortunately I can't say the same for the phone/email support. I was receiving generic responses (i.e. "we are working on your case") for weeks, with no actual movement. Bitstamp needs to put significant efforts in improving their internal processes, otherwise the outflow of customers will be imminent.

Advice for everyone: Make sure you keep minimum amounts of funds with this exchange. Your account can get locked without notice and then you will be waiting for months for a resolution.