This question already has an answer here:
I have geoserver 2.5.1 running on ubuntu 14.04 server. I tried to compose an image mosaic with ImageMosaicPlugin but I always get a message Could not list layers for this store, an error occurred retrieving them: Failed to create reader from file:data/dalekovod and hints nul. Images are in the right file format because geoserver recognizes them, and I can load each at one of them separately but I have 120 images.
Any idea what could be the problem?