The English translation of the Basic Law (a document that is for all intents and purposes an long term interim constitution) states in Section 5, Article 54, Clause 2 (emphasis mine):
The term of office of the Federal President shall be five
years. Re-election for a consecutive term shall be permitted
only once.
This wording appears to state that the President of Germany can be elected to an unlimited number of terms but can not run for a third consecutive term (i.e. they must sit out for a term before being able to be elected again).
Link to the English translation in PDF form
English translation from the German Cabinet's website