Thus far, you are accurate in every aspect.
One potential solution is to extract the AdBlue fluid and substitute it with a new batch of high-quality fluid. Additionally, using an anti-crystallising chemical may be beneficial. The probability of achieving success is not substantial, but considering the expense, it is advisable to give it a try.
If it doesn't work, you will need to purchase a new tank and sensor, and hope that it resolves the issue.
Undoubtedly, some individuals here may suggest a 'AdBlue erase' option. However, it is up to you to carefully consider this choice and its potential legal and insurance consequences.
Additionally, if you made a purchase of the vehicle from a trader during the last 6 months, it is advisable to return it to the original place of purchase.