Cannot receive reward for field-research

Today I am on the last day of the field-research quests.

I have already collected about 8 stamps today but I still cannot receive the reward: nothing happens if I click on the big orange button.

If I close the window it jumps back to "You need 1 more stamp to receive ....". I already tried to disable / enable WiFi and to restart the game multiple times, with no effect.

Anyone got an idea what could I do ?

Based on the discussion within the comments, it would appear your storage is full. According to this reddit post, you are unable to collect field research rewards with a full inventory. It would be safe to assume this accounts for both Pokémon Storage and Item Storage.

The solution to this is to transfer Pokémon and/or drop items to make room for your rewards.