Error 503, only with my Mac and iPhone?

Hello.
I’ve a problem that is driving me crazy. I got the last WP version. When I open up an article to edit it (even if I create a new one), when I try to upload an image I get:

503 Service Unavailable
The server is temporarily busy, try again later! Powered By LiteSpeed Web Server
LiteSpeed Technologies is not responsible for administration and contents of this web site!

Or, If I’m VEERY LUCKY, like 1% of total cases, I get correctly the upload screen but when I try to take every action (upload an image, click on “library” to see Images I already uploaded etc.) I get a blank screen. After getting that blank screen, when I start moving in any page of wp-admin I keep getting this white page. After something like 1-2 minutes, the wp-admin starts working again.

But I get this only with my Mac and iPhone… other computers work beatifully! I can’t get out from this! They worked time ago, now suddenly I got this problem!

This is what i’ve done up to now:
– Optimized /Repaired DB Tables
– Tried to change Theme
– Tried to change Browser
– Deactivated all the plugins

And still no luck! My host of course said it is a problem of mine (basically thats it.. but I get it only with Apple devices!)

Hope this happened to someone out there and can help me out. I can’t go on without uploading images! Thanks in advice!