Optiphen - INCI: Phenoxyethanol (and) Caprylyl Glycol - is, unfortunately, a bit of a pain in the bum when it comes to lotion making. Yep, it's another reason your lotion could fail.
As a note, I'm hearing that Optiphen Plus (INCI: Phenoxyethanol (and) Caprylyl Glycol (and) Sorbic Acid) and Optiphen ND (INCI: Phenoxyethanol, Benzoic Acid, Dehydroacetic Acid) are curdling lotions as well, so this may apply to that product as well.
Kuldip suggested: Add optiphen at about 45-55°C, the trick is to keep mixing until cool. Usually adding optiphen to a cationic emulsion the product sometime thins out and will eventually thicken as it cools.
I have no experience with Optiphen - as you all know, I'm a liquid Germall Plus girl - so I can't make any other suggestions. If you have any, please share them with your fellow formulators in the comments below!