Can't cast copied spell - 1.12.1 vmangos

[Ayos]

Hi!
My problem is, that I can't cast newly copied spell variants.
My steps so far:
- Copied an existing spell to a new & unused ID without changing any other columns in SQL spell_template.
- Did the same in Spells.dbc, with the same spell & new ID.
- Copied the dbc to the server and to the client via a patch.
- Restarted the server, and cleared client's cache.

Also added the spell to a talent tree, where it is showing perfectly, and I was able to learn it.

But when I try to cast it, it just casts infinitely.
The spell I copied this from works perfectly.

What did I miss?
Any kind of help would be highly appreciated!