My preference is usually: ‘hi, thanks for creating this tool/library/whatever. It really makes a meaningful difference in my life. I’m encountering a problem with the blah blah module, which you can repro thusly.’

Don’t grovel, but also acknowledge that the only compensation the reader of the report is likely to get is in the form of these extremely rare moments of gratitude.

I should also mention that I maintain an open source app that gets used by the public, and is regularly confused by them with being a product of a group of paid individuals. In fact, they usually think the software is a result of their tax dollars at work. I’ve received a lot of really negative, shitty feedback from all sorts of people, including folks in the software industry (shout out to everyone who ever emailed me from their Amazon.com work addresses). Don’t be like these people. The ones who start by simply saying “thanks” not only get more from me, but also help keep my motivation up.