spamassassin (or spamd) fails while accessing bayes databases - R/O: tie failed: Transport endpoint is not connected

just found lines like these in the spamd.log (i.e. spamassassin was not able to use the bayes databases any more...)

warn: bayes: cannot open bayes databases /.../.spamassassin/bayes_* R/O: tie failed: No such file or directory
...
warn: bayes: cannot open bayes databases /.../.spamassassin/bayes_* R/O: tie failed:
warn: bayes: cannot open bayes databases /.../.spamassassin/bayes_* R/O: tie failed: Transport endpoint is not connected

the problem seemed to have been a mismatch in used versions of the BereklyDB files (don't know how exactly they got mixed up...)
anyways, the following commands helped:

for i in auto-whitelist bayes_*; do db4.6_dump ${i} | db4.3_load ${i}.db; chown Debian-exim:Debian-exim ${i}.db; chmod g-rwx ${i}.db; done;
mmv '*.db' '#1'

after that one may check/sync/rebuild the databases along the lines of:

sudo -u Debian-exim sa-learn -u USERNAME --rebuild --dbpath /.../.spamassassin/ -D

some inspiration cam from this thread: http://lists.debian.or.jp/debian-users/200601/msg00113.html