Again, its a UTF-8 / character problem
Thanks to the excellent phone support we could figure out the problem. The second gallery used titles with French letters, like Ardèche or Chârtreuse. The use of one single letter like this caused the whole gallery to disappear! As a quick-and-dirty workaround, one could encode some letters, like the old fashioned ß for ß or ä for ä. It then displays wrong in the admin area, i.e. "Straße" instead of Straße. But on the public gallery page it will display correctly.
This is rather unsatisfactory. It excludes non-encodable characters completely. Please fix this asap!