@dollhauss I’ve taken a look into it and it’s obvious the plugin has been altered most likely by the hack you had. Other plugins you have there are likely affected too. You might need to do a deep cleaning and re-install most (if not all, depending on the hacking size) of the plugins.
The error you’re getting when you activate Asset CleanUp is due to extra code (the hacking one apparently) that is placed above the “WpAssetCleanUp” namespace. The file Preloads.php has the namespace right after the <?php tag as it should have it. Take a look here at the original source of v1.3.5.7: https://plugins.trac.www.ads-software.com/browser/wp-asset-clean-up/tags/1.3.5.7/classes/Preloads.php (compare it with the version you have, be careful when downloading / checking as any anti malware software you might have could detect it and automatically send it to quarantine, it happened to me when I was dealing with hacked websites).
If you delete your version of Asset CleanUp and reinstall it, you shouldn’t get the same error. I’d advise you to contact your hosting company (in case you haven’t already) to give you details about the hack. The might have malware scanners available too. Also, I’d recommend you plugins such as https://en-gb.www.ads-software.com/plugins/better-wp-security/ (not affiliated, I’ve tried it and it worked well for me) to add further security to your website.
Let me know if re-installing the plugin fixes it (unless you have a hack that immediately alters PHP file from just installed plugins) and hopefully, you will get your website cleaned soon. More exciting features are coming up for Asset CleanUp ??