Before I waste too much time digging in the wrong place, we have an app running on Clojure 1.10 Beta 4 and when we try to report an exception to the log file (with Timbre, which in turn uses Aviso "pretty"), we get this instead

#error {
 :cause "clojure.pprint.proxy$java.io.Writer$IDeref$PrettyFlush$4923d848 cannot be cast to clojure.pprint.PrettyFlush"
 :via
 [{:type java.lang.ClassCastException
   :message "clojure.pprint.proxy$java.io.Writer$IDeref$PrettyFlush$4923d848 cannot be cast to clojure.pprint.PrettyFlush"
   :at [clojure.pprint$write$fn__8503 invoke "pprint_base.clj" 233]}]
 :trace
 [[clojure.pprint$write$fn__8503 invoke "pprint_base.clj" 233]
  [clojure.pprint$write invokeStatic "pprint_base.clj" 233]
  [io.aviso.exception$format_property_value invokeStatic "exception.clj" 487]

It seems to work just fine in the REPL -- but my debugging is hindered by part of the stacktrace being omitted (the key part: between my code and the actual, failing call into Timbre).

Is any of the clojure.pprint.PrettyFlush stuff new in the 1.10 branch? (and thus might cause this)

looks like maybe a weird class loader error?

Yeah, just not quite sure which path to start down...

Nothing changed around here that I recall

It almost looks like that’s an exception when printing in aviso

Have you tried without it?

I've tracked it down to only happening in the JAR version of the project -- when run from source, it all works.