I had problems with them Cobalt Strings, too. It was the .055 - .110 set.
Two times the G-string broke when stringing the bass with a new one. Every time on the same place, where the string gets thinner and the violet wrapping starts. Right after the saddle, where the string is "in the air".
I have to make a compliment to Thoman Germany, they sent me a spare string in no time. The second broken string was discussed with Musik Meyer, the German distributor of Ernie Ball. It lasted a bit longer, but they sent me a new one.
This third string now seems to be better. OK, I strung it carefully as a hedgehog trying to make little ones. No more problems since then.
I purchased two sets of Slinky Cobalt Flats in .045 - .130 for a 5-banger P-Bass (Vychodil P5), and had no problems when stringing it. They sound great, and my SR 5 Classic is waiting to get strung with the same strings. As soon as one of the Elixir Nanoweb strings that are on this bass will fail. This may take some time.