Current geoserver seems not to be able to create an Image Mosaice from one or more *.asc files
After first having some problems using my own asc files, see (search for 'shp index of mosaic of asc files fails to fill'):
I ended up to test just the precip30min.asc and precip30min.prj files that are provided as example asc files with Geoserver. All WMS requests to this store/layer fail.
copy precip30min.asc and precip30min.prj (from geoserver coverage data dir) to some dedicated directory
create an Image Mosaic store for it
note that a valid index.shp is being created (with in my case the valid bbox's)
create a default layer from it
the go to Layer Preview and try to view your layer as openlayers wms
see the error that is viewable in attached image
I first had the impression that it had something todo with my data being nog epsg:4326 and some resampling was faulting. But as the above simplest test is not even working, I think there is more.
In the mailinglists are some stacktraces, plz let me know if needed here.
Happy to test/help/debug, or provide (other) data, please sent and email to firstname.lastname@example.org
Downloaded selfcontained and in debugging session too