Police in the northern city of Hamburg said a venue where Turkey's Foreign Minister Mevlut Cavusoglu had planned to speak on Tuesday could not be used "after serious fire safety deficiencies were identified".
A bitter row has flared since late last week after three other local German authorities banned Turkish ministers' scheduled appearances ahead of the April plebiscite on a plan to scrap the prime minister post in Turkey.
The other German authorities had cited capacity problems in hosting the events, which they said were likely to attract large crowds.
President Erdogan at the weekend angrily labelled those bans "Nazi practices", ignoring Berlin's claims that the federal government did not have a hand in the decisions.
Merkel's office on Monday rejected Erdogan's claims as "absurd and out of place" but also sought to draw a line under the dispute, calling for all sides to let "cool heads prevail".
However, the latest decision looked set to reignite the conflict after Cavusoglu had on Monday also accused Berlin of working to scupper the rallies.
"The hotels, the conference halls are being put under pressure, police are being sent to the owners of these places: it's systematic pressure, a manoeuvre aimed at erecting systematic barriers," he said.
"The state and the state-run institutions are all implicated. That's unacceptable. As for us, we will take the necessary measures, we have no fear of anyone," said Cavusoglu, who was scheduled to meet his German counterpart Sigmar Gabriel on Wednesday.
Hamburg police said officers had Monday inspected the Plaza Event Center site in a southern harbour district of the city and, after discovering the fire safety flaws, "immediately banned the use of the property".
"Not the event was cancelled, but the use of the Plaza Event Center," a police spokesman told AFP.
With some 1.4 million Turks in Germany holding the right to vote in Turkey, it is not uncommon to see politicians travelling from Ankara to try to sway voters.