These things can happen. FPC is not error free, and sometimes internal errors occur. An internal error in a release version is always report worthy, unless already reported. (and even then it might we worthwhile to check if there already is a reproducable case)
In this case, if Thaddy had bothered to check mantis instead of a knee-jerk reaction, he would have found
https://bugs.freepascal.org/view.php?id=32352 right on the first page with errors.
The trick then is to minimalize it to a fairly small source (e.g. a .dpr and one or two units max) without lazarus. Typically you start to cut source code and used units till the error doesn't occur anymore. If that happens, restore the last change, and cut something else etc.
No reproduction yet. Even a small lazarus project without much dependencies will be accepted, and I will try to reproduce it myself.