Hi Amon,
Can you try
1. Creating a new table with one column of DECIMAL datatype.
2. Building an Attribute View on top of this.
3. Having a Calculated Column with TIMESTAMP datatype and the formula from my last post.
If this works then it could be an error with other Attributes involved in the Attribute View otherwise there is some error with what you're trying to do.
I will need more details of the attribute view that you're trying to build to debug that error.