This happens to me with Ilford developers often. The remedy is to pull off the outer (black) part of the cap, not easy - this part works as a child proof cap - and unscrew the exposed (white) cap beneath.
Darko, the Adox cap on your picture looks exactly the same as the one on their larger 1.2L bottles which gave me a lot of trouble years ago when trying to open their ADOTOL WA ans NE bottles.
I wonder how good Tetenal bottles are as I have notice they have changed them. The old ones were very good, caps as well.