Hi Andrei,
I can't say I remember calling a drop table from within a transaction, but I just tried it with the same versions as you have above (on redhat 9), and at least for a test table with two rows, droping the table within a transaction worked fine.
I don't remember seeing those compiler warnings either - do you have the version of nspostgres as referred to by:
https://openacs.org/doc/openacs-5-1/aolserver4.html
?
