@andrea_r:
Thank you for the reply. I have asked and also researched them and the answer I got every time is that they do support wildcard subdomains.
I am still confused on what I actually did wrong. Was it the part where I had to “replace all WordPress lines” in .htaccess with what they provided me in the Admin panel?
There is also this thread and the original poster also uses Just Host with wildcard subdomains, in which you asked the same thing:
https://www.ads-software.com/support/topic/can-i-use-wp30-with-multisite-function-running-from-a-subdirectory
