Sorry to hear about your problem with the Fresenius tubing. A decade ago, there was a recall of Baxter tubing that caused hemolysis due to a narrowing of the defective tubing. We have not hear any more details about this problem you described with the Fresenius but it sounds dangerous as well. Usually the connectors you described are the Luer syringe type connectors and they can be either too tight or too loose. If they are loose on the low side, air can get in and if loose on the high side, it may leak. On the other hand, if they are too tight, they may break when you attempt to loosen it, so you have to be careful as well not to over-tighten the connectors.
http://en.wikipedia.org/wiki/Luer