From what you describe, it sounds like your custom master page may not
be accessible by SP. If you successfully create the site/collection
but you see the file not found error, SP can't render the page you're
after becaeuse it's missing the all important master page. Not totally
sure that's your issue, but it sure sounds like it.
Update your web.config StackTrace=TRUE you will see the real error of what file is missing.
Which master page is attached to the site template ?
Or in other way on the source site did you also modified the master page ?
make sure while you create a new site using that template it will be inherited from the source master page.
Also make sure if there is any custom feature used on source site it should be activated on the destination portal and then try to make site collection.
Or Try this solution: Go to Site settings of parent site -> under group Look and Feel, click on Master Page -> check on 3 checkboxes: Reset all subsites to inherit this Site Master Page setting Reset all subsites to inherit this system Master Page setting Reset all subsites to inherit this alternate CSS URL
Click OK Check your sub site again.