I just played Freedroid after a loong time, and - after I quit and looked at the
terminal window ran into this:

FreedroidRPG 0.15-1461-g49e68f7 encountered a problem in function: add_obstacle
Invalid obstacle (iso_oil_7.png) position on level 17: t318 x91.27 y39.92

I do not know where, when or why this occurred. The message written to the
terminal suggests to report it as a bug - so here we are... :)

Is this message of any use without further context (which - unfortunately - I
cannot provide)?

Yeah, that's a recent bug, we are aware of this.
I tried to hack together a patch http://rb.freedroid.org/r/2173/ , not sure if
it's good though.

Linked review request was discarded because https://bugs.freedroid.org/b/issue602  was resolved. Is this still an issue?

Was indeed solved by the fix of issue602.