pg_upgrade from 9.5 to 9.6 fails with "invalid argument"

Hello,

for some reason pg_upgrade failed on Windows 10 for me, with an error message that one specifc _vm file couldn't be
copied.

When I try to copy that file manually everything works fine.

After running a "vacuum full" on the table in question the upgrade goes through.

One thing I noticed in the --verbose output of pg_upgrade is that the old cluster - despite being a 9.5 one - has the
"pg_controlversion number 942" 

Here is the part of the pg_upgrade output:

.....

copying "d:/Daten/db/pgdata95/base/16410/85351" to "d:/Daten/db/pgdata96/base/16411/85351"
   d:/Daten/db/pgdata95/base/16410/85351_fsm
copying "d:/Daten/db/pgdata95/base/16410/85351_fsm" to "d:/Daten/db/pgdata96/base/16411/85351_fsm"
   d:/Daten/db/pgdata95/base/16410/85351_vm
copying "d:/Daten/db/pgdata95/base/16410/85351_vm" to "d:/Daten/db/pgdata96/base/16411/85351_vm"
   d:/Daten/db/pgdata95/base/16410/85358
copying "d:/Daten/db/pgdata95/base/16410/85358" to "d:/Daten/db/pgdata96/base/16411/85358"
   d:/Daten/db/pgdata95/base/16410/85358.1
copying "d:/Daten/db/pgdata95/base/16410/85358.1" to "d:/Daten/db/pgdata96/base/16411/85358.1"
   d:/Daten/db/pgdata95/base/16410/85358.2
copying "d:/Daten/db/pgdata95/base/16410/85358.2" to "d:/Daten/db/pgdata96/base/16411/85358.2"
   d:/Daten/db/pgdata95/base/16410/85358.3
copying "d:/Daten/db/pgdata95/base/16410/85358.3" to "d:/Daten/db/pgdata96/base/16411/85358.3"
   d:/Daten/db/pgdata95/base/16410/85358_fsm
copying "d:/Daten/db/pgdata95/base/16410/85358_fsm" to "d:/Daten/db/pgdata96/base/16411/85358_fsm"
   d:/Daten/db/pgdata95/base/16410/85358_vm
copying "d:/Daten/db/pgdata95/base/16410/85358_vm" to "d:/Daten/db/pgdata96/base/16411/85358_vm"

error while copying relation "public.wb_downloads" ("d:/Daten/db/pgdata95/base/16410/85358_vm" to
"d:/Daten/db/pgdata96/base/16411/85358_vm"):Invalid argument 
Failure, exiting

The file in question is 65.536 bytes in size.

I saved all log files and the complete output from the failed run, so if you are interested I can supply them (I ran
pg_upgradewith the --retain option). 

Regards
Thomas