Comment 9 for bug 1868502

I tried to narrow down the problem by re-building unison with OCamL 4.05 on my Focal (20.04) client (no ocaml deb package installed):

- Downloaded OCamL 4.05.0, ran make and make install
$ which ocamlopt
/usr/local/bin/ocamlopt
$ ocamlopt -v
The OCaml native-code compiler, version 4.05.0
Standard library directory: /usr/local/lib/ocaml

- Downloaded sources of unison 2.48.4, ran make ('make world' or 'make world opt' didn't seem to make any difference)
$ ls -l ./src/unison
-rwxr-xr-x 1 meinhard v5 5038200 May 20 14:50 ./src/unison
$ file ./src/unison
./src/unison: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=cddb0691b6b9721087be3f2f6f4f2397f57f4171, for GNU/Linux 3.2.0, with debug_info, not stripped
$ ldd ./src/unison
 linux-vdso.so.1 (0x00007ffda03f4000)
 libutil.so.1 => /lib/x86_64-linux-gnu/libutil.so.1 (0x00007f97aa540000)
 libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f97aa3f1000)
 libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f97aa3eb000)
 libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f97aa1f9000)
 /lib64/ld-linux-x86-64.so.2 (0x00007f97aa8b8000)
$ ./src/unison -version
unison version 2.48.4

Yet no luck when syncing to my Bionic server, even though the error message is very different:
$ ./src/unison <target>
[...]
Looking for changes
Uncaught exception Failure("input_value: ill-formed message")
Raised at file "/data01/meinhard/Scratch/Software/Unison/unison-2.48.4/src/lwt/lwt.ml", line 126, characters 16-23
Called from file "/data01/meinhard/Scratch/Software/Unison/unison-2.48.4/src/lwt/generic/lwt_unix_impl.ml", line 102, characters 8-23
Called from file "/data01/meinhard/Scratch/Software/Unison/unison-2.48.4/src/update.ml" (inlined), line 2096, characters 2-69
Called from file "/data01/meinhard/Scratch/Software/Unison/unison-2.48.4/src/uitext.ml", line 700, characters 16-56
Called from file "/data01/meinhard/Scratch/Software/Unison/unison-2.48.4/src/uitext.ml", line 788, characters 6-90
Called from file "/data01/meinhard/Scratch/Software/Unison/unison-2.48.4/src/uitext.ml", line 810, characters 19-66
Called from file "/data01/meinhard/Scratch/Software/Unison/unison-2.48.4/src/uitext.ml", line 870, characters 21-43

Not having any clue about OCamL, I don't know what causes this error - the source file lwt.ml doesn't look suspicious in any way at the incriminated line.

Any help I can further provide to understand and fix the problem, please let me know. (For the moment I'll keep OCamL 4.05 and unison 2.48-4 sources around...)