The data couldn't be read because it isn't in the correct format.

I'm getting this error inside an iOS app which is connected to the nginx server, and it is preventing users from accessing the service. After rebooting the server a few times, it goes away by itself, then comes back a few days later. What could be causing this. I researched all possible solutions online and it seems to the related to the use of "\n" in a string at times, but I don't have such issues with my code. Thanks for your your help.