It appears that your server is having problems. That error is being generated by the web server itself, not by concrete5. Possibly it is overloaded or too busy serving other requests. If you're using a budget hosting operation, you might want to consider one that has the resources you need. concrete5 does require a fairly generous server environment.

Hi jero, thank you for you reply.

In this case, could you tell me the minimum server and hosting requirements for a website with 1500+ pages and 4500+ images to run without issues? The website also receives high traffic, sometimes 20k-40k visitors per day.

If you're using a budget hosting organisation and paying only a few dollars a month, you won't be getting much performance for the money. My guess would be that you'd want your own VPS server for this, preferably from a reputable vendor that doesn't oversell the number of virtual machines. I'd look for a VPS with at least two processors and a couple of GB of RAM - probably a "Medium" VPS by most standards. It would also be interesting to know the URL of your site - I can then give you a rough idea of how many other websites are hosted on your web server and what sort of server software is in use which sometimes indicates whether the hardware is being stretched.

Hi Jero, thanks for your reply.

Our website is calledhttp://www.exrx.net ,
and I reported your message to our host provider and asked him if he can upgrade our server to your suggestions, his reply was:

"This server is probably 12x more powerful then a VPS. This machine could be used to host a few dozen VPS's if I load it full with memory. I can accommodate if you want to just throw more resources at this, even though the server still has a very light load, but that will be costly."

I'm a bit confused now, because you said our server is not good but the host provider says the server is powerful and our website has a light load.

Well maybe if your hosting provider is confident that you are on a machine that's fit for purpose they can explain why you are seeing 503 errors. 503 means that there is a lack of resources available - seehttp://httpstatuses.com/503 for more information. It does sound like a server resource issue. I've only ever seen this error on machines that are being hammered.

Your website doesn't seem to be running concrete5 - the source looks like static HTML. There are two domains listed for your IP address, so this is either a VPS or an extremely lightly loaded host. It's Apache, rather than Litespeed which is typically used to squeeze more performance out of budget hardware (IMHO)