Otherwise, looking at why the package.cache is out of date, along with what tests were run around the time that it happened, might give a clue as to what went wrong. I think I got it. After running 'make maintainer-clean' in the validation tree I still have this files in testsuite left from previous validation:
tests/cabal/local01.package.conf/package.cache tests/cabal/local03.package.conf/package.cache tests/cabal/local03.package.conf/testpkg-1.2.3.4-XXX.conf tests/cabal/local03.package.conf/testpkg-2.0-XXX.conf tests/cabal/local03.package.conf/testpkg-4.0-XXX.conf tests/cabal/local04.package.conf/newtestpkg-2.0-XXX.conf tests/cabal/local04.package.conf/package.cache tests/cabal/local04.package.conf/testpkg-1.2.3.4-XXX.conf tests/cabal/local05a.package.conf/package.cache tests/cabal/local05b.package.conf/package.cache tests/cabal/local06.package.conf/package.cache tests/cabal/local06.package.conf/testdup-1.0-XXX.conf tests/cabal/local06.package.conf/testpkg-1.2.3.4-XXX.conf tests/cabal/localT1750.package.conf/T1750A-1-XXX.conf tests/cabal/localT1750.package.conf/T1750B-1-XXX.conf tests/cabal/localT1750.package.conf/package.cache tests/cabal/localshadow1.package.conf/package.cache tests/cabal/localshadow1.package.conf/shadow-1-XXX.conf tests/cabal/localshadow1.package.conf/shadowdep-1-XXX.conf tests/cabal/localshadow2.package.conf/package.cache tests/cabal/localshadow2.package.conf/shadow-1-YYY.conf tests/cabal/package.conf.ghcpkg02/package.cache (plus lots of other build artifacts in the testsuite). Deleting files listed by 'git ls-files -o' solves the problem. I wonder how is it possible that - if this is really the cause - I am the first person that run into this problem? Janek