Here the possible reasons for this kind of error with the possible fixes:
Probably permissions or a deny rule, check your .htaccess file.
- It’s may be due to the reason that WordPress is trying to load the certain directories but those directories doesn’t exist on that URL. May be directories get deleted accidently. So, it cannot open those directories because they do not exist.
- Check that what changes have you done to the site recently and how did this start happening exactly, have you transferred it, have you added plugins, etc.?
- you can just restore a backup from when it was working last, that should fix it
- It can be the directories permission issue from your host. Go and talk to your hosting support. They will help you for sure if you are using a good host like Siteground or NameCheap etc.
- You might want to check PHP version as well, sometimes, for example, if your host change it to 7.4, if your site was using older than 7.4 it’s probably because of that as well.
- One last suggestion is to check the permalinks from wp-admin > Settings > Permalinks – just click save without doing changes, sometimes they need a reset like this which fixes page issues. I’m here in case something happens and further assistance is needed and hope this helps!