500 Internal Server Error with website on mobile

For several months my website has been giving an "500 Internal Server Error" when attempting to open the home page, or for that matter, ANY pages under it's domain. This happens only on an iPhone mobile browser -- Mac, Windows and iPads do not have the problem (not sure about about Google devices).

My ISP says it is probably a problem with the web design software (I used WD7) as it uses fixed page sizes and not "cascading sheets which are more mobile-friendly" (direct quote -- I am not completely sure what this means).

I am not sure what to do. One problem is that several other sites hosted by the same ISP have the same issue......as well, I have a Wordpress site and a CoffeeCup-designed site under the same domain, and those pages have the same problem, cannot be opened.

Any insight as to why?

Not sure. It could be a server issue.

I'm sure one of the more tech savvy members will have a more enlightened answer.