Hi Mprivat,
hier kommt die Antwort, allerdings in English:
While in the U.S., the traveler may go to Canada, Mexico or the Caribbean and adjacent islands and re-enter the U.S. using the visa stamp or I-94 entry admission notation he/she was issued on the VWP passport when he/she first arrived in the U.S., although the time spent there is included in the overall 90 days allotted for the visit. Consequently, it is important to note that short visits to Canada or Mexico will not result in the issuance of a new U.S. entry document providing a "fresh" stay of 90 days.
The terms of the VWP are very clear - it is only to be used for occasional, short visits to the United States. If a U.S Customs and Border Protection ("CBP") officer believes that a traveler is trying to "reset" the clock by making a short trip out of the U.S. and then re-enter for another 90-day period, he/she can be denied entry. In this case, the traveler will have to obtain a visa for any future travel to the U.S. in order to be re-admitted to the U.S. To re-enter the U.S. shortly after a previous admission expired, the traveler will have to convince the CBP officer that he/she has a bona fide purpose for the new entry, and should be documented accordingly.
ESTA ist also nur für Kurtaufenthalte bis zu max. 90 Tagen gedacht und geeignet. Man muss schon einige Zeit verstreichen lassen, bevor man wieder einreisen darf. Bei einem kurzem Zeitabstand ist eine glaubwürdige Begründung erforderlich. Liebeskummer zählt wohl nicht dazu. Bei mir waren es einmal eine Privat - und danach eine Geschäftsreise und die Begründung hat man akzeptiert. Also, wer öfter und länger in die U.S.A. reisen möchte, benötigt ein Visum