Please Help Bulk Convert Not Working

Hi, i have setup Webp Express and its working fine, converting new files. But i have too many old images that i want to convert, but when i click on bulk convert it takes forever. and nothing happens.

In the console i have this

VM212:1 Uncaught SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at Object.success (bulk-convert.js?ver=2:21)
    at i (load-scripts.php?c=1&load[chunk_0]=jquery-core,jquery-migrate,jquery-ui-core,jquery-ui-widget,utils&ver=5.4.1:2)
    at Object.fireWith [as resolveWith] (load-scripts.php?c=1&load[chunk_0]=jquery-core,jquery-migrate,jquery-ui-core,jquery-ui-widget,utils&ver=5.4.1:2)
    at x (load-scripts.php?c=1&load[chunk_0]=jquery-core,jquery-migrate,jquery-ui-core,jquery-ui-widget,utils&ver=5.4.1:4)
    at XMLHttpRequest.c (load-scripts.php?c=1&load[chunk_0]=jquery-core,jquery-migrate,jquery-ui-core,jquery-ui-widget,utils&ver=5.4.1:4)