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The problem is that your files have wrong owner and there for you dont have permissions. Where do you host this? Is it shared hosting or? Ask your hosting to fix this.
Oops, Opening file for writing failed on error fopen(/home/www/a1d16a19b830dc7ecf5b30122df3bd09/web/V5/templates/rt_flux/custom/css-compiled/flux.css): failed to open stream: Permission denied.
How do you run your php. Is the apache owner of the files or ?