I can't add artifacts from other sources than Basic Rules even though I have access to them. Also can't find them as templates for homebrewing. Any idea why this is happening?

We recently had a pause of about a day for the content sharing, could that have caused this?

You can't use an item as a homebrew template unless you have bought it. Sharing is not enough.

Is the character you are trying to add the artifact to in the campaign with sharing turned on?

Well, that explains it then.

I could add it to the characters in the campaign, just checked.  

Quote from Lumenar89 >>

I could add it to the characters in the campaign, just checked.  

 
You only have access to the Character Builder/Sheet content that is being shared to you while using a character inside of the campaign.