"I worked for Charity X for 19 months. I've decided to pursue other opportunities".
Employers (HR departments) aren't particularly bright, but they are easily triggered. Don't even hint at "difficulties". I have been an expert (retired now, don't put full time into maintaining expertise), and had to help HR depts at several companies (medium to large) evaluate resumes in my field. Their first step is to discard resumes for any excuse (misspellings, incorrect sentence structure, any perceived imperfection at all), without regard for the content of the resume. This reduces the Unmanageable Heap of Resumes to a Pile, which is easier for HR to handle.
Your new (potential) employer has no need to know about your previous employer's problems or internal state. Depending on where you are, and what industry you're in, it could be restricted.