As a middle-tier master, his successes in chess were few. It was not until 1956, in the Netherlands, that Diemer finally enjoyed a true success, winning the Reserves Group of the Hoogovens tournament and later the Open Championship of the Netherlands.

Alas, his success in the Netherlands was not to be repeated, as Diemer became less interested in chess, and increasingly interested in Nostradamus, the famous 16th century French clairvoyant. He believed that he had cracked the great seer's secret code, and during a period of 25 years he is said to have mailed over 10,000 letters on this subject. In 1965 he was committed to a psychiatric clinic. The doctors considered that chess was too much of a strain for Diemer's nerves and they forbade him to play the game. In six years this order was recinded, and Diemer, while no longer in form, nonetheless took great enjoyment in his return.