apologies if the UI is not very organized, it was built incrementally without much planning 🙁
If you set the fixed fee to zero, then it will be reset to the minimum allowed, which is 30k satoshi. This is because transactions would not be able to go through if the withdrawal fee is not covered.
As a starting point, set the fixed withdrawal fee to 30k satoshi, and the fixed fee to zero.
Once you can confirm that this works, you can then increase the values if you wish.
Do let me know if you continue to experience problems.