I have migrated from a working version 4 and found this issue via Health check, have tried everything to clear this, i have looked over the file permissions in the docs but this was a bit unclear and I figured there was a simple solution as V4 i never had any permission issues. This issue happens on every pdf, invoice, recurring, and even deleted pdf.
Most likely you have queues configured under a different web user, the fix here is to chown the public/storage directory recursively to the webuser.
I have done this again, and it cleared the error using:
chown -R webuser.psacln /var/www/vhosts/sitename.com/httpdocs/billing1/public/storage, then i setup a new recurring invoice and yet another error,
I think the first thing needed, is to get the queue’s running as the webuser. it looks like a diffferent user is writing files to your directory. the entire public/storage and storage directories should be resursively chowned by the web user.