Your system volume is almost full and you need to free up disk space. Or you simply no longer need those tens of thousands of previews you once generated.

By default all previews are stored in the following folder:

Presstore 4 (P4):

/usr/local/aw/conf/index/archive/<name_of_your_archive_index>/1

Presstore 5 (P5):

/usr/local/aw/config/index/archive/<name_of_your_archive_index>/clips

You can safely delete the contents of those folders in order to free up disk space. Please do not delete the '1' or 'clips' folder. The archive index will continue to contain the links to those previews. However, those links will simply point nowhere.