Warning: trim() expects parameter 1 to be string, array given in _minify_construct_min_js_path() (line 321 of ../minify.admin.inc).

Minifying js files is giving on the very first attempt:

Warning: trim() expects parameter 1 to be string, array given in _minify_construct_min_js_path() (line 321 of /home/username/public_html/sites/all/modules/contrib/minify/minify.admin.inc).

But then the warning message is gone after refreshing the page admin/config/development/performance/minifyjs.