Thanks - I don’t think it is me accessing the file too frequently, as the problem usually occurs upon the first file access on any particular day.
Did you mean to follow the Blog that you have shown here?
Thanks
Yup,
Hello everyone.
At last, I have solved the mystery.
It was, in fact, caused by my .htaccess file, as was suspected by some people here.
The reason that I previously discounted that as the cause was because I had been reading cached files, after removing the file from the site. So I continued to receive the same error. Consequently, I thought that the .htaccess file was not the cause of the problem. A big mistake 
The file had a rule in it that prevented access from any request that did not have my domain name as a referrer. It was a rule I had forgotten to remove.
Thank you to everyone who helped. It is much appreciated.
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No Problem!
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