Error when trying to add a new field: An error occurred at ... Error description: 0

Hi,

when i try to add a new field to a view the following error message appears:

An error occurred at http://www.project.com/admin/build/views/ajax/add-item/name_of_view/default/field.

Error description: 0

This issue just appeared - today I worked on some views without problem but now this message appears and I cannot do anything - every time I want to add filters, fields, arguments or even want to change setting in existing fields this message appears.

Ok - now I tried another browser. Usually I use Firefox. Then I tried with Chrome - everything worked fine. Then I tried again with Firefox - everything works fine... has somebody an idea what can cause this problem?

Best,
Tobias