This error is most likely because you are mixing incompatible version of oacs and dotlrn. Either you can use oacs-4-6 (which happens to be the version that the tarball is based on) together with dotlrn-1-0, or HEAD from both.
Did you really follow http://openacs.org/projects/dotlrn/dotlrn-doc/dotlrn-installation, especially the paragraph where it says:
"
If you are installing OpenACS, follow the extensive installation documentation. Stop at the point where the OpenACS installation instructions tell you to "download OpenACS". Don't use the "Quick Downloads"! Continue on with this document.
cvs -d :pserver:anonymous@openacs.org:/cvsroot login
(just hit return for the password)
cvs -z3 -d :pserver:anonymous@openacs.org:/cvsroot checkout acs-core
"?
Also have a look at the comment at the bottom of the installation instructions.