Is is ok to use slightly sour curd for chicken marination?
There are types of curd* that are supposed to be sour.

If the kind you are using is not supposed to be sour, then do not use it if it is. Expiry date is not everything. How it was stored in shop plays a big part in food safety, and dairy products that soured uncontrollably are safety risk.

You can make sour dairy products in home if you know what and how you are doing, and they will be safe to eat, but that's it. If it soured not on purpose, throw it out. Maybe you can also change shop you get your food from, or send it to your local food safety agency for examination. If there is one, they can examine your curd, inspect shop, and fine shop owner if souring was due his neglect.



* Note, English is not my primary language and what word means what gets blurry when it comes to Polish-English translation.