As we have customized the SharePoint Rich Text Editor (RTE) by adding additional buttons and assets, I suspected our own extensions to be the reason of the error at first. Unfortunately, after removing the customization, the error persisted.
The error itself was quite odd: we got it while using the Content Editor Web Part only. While editing content using exactly the same RTE as a Rich HTML control there was no error. Furthermore, we received the error in the test environment only. Our development machines didn't show up any error at all!
Looking at the reason of the error and the differences between the bform.js files we have found out that it has to have something to do with different SharePoint installs we we using. While we still use the updated RTM on our development machines, we have used the slipstreamed SP1 version of SharePoint on the test environment.
Update April 16, 2008:
Replacing the BFORM.JS only doesn't solve the whole issue. While editing the content using the RTE in the Edit Form, SharePoint keeps throwing the error on each key press. In order to fix it, you have to replace the LAYOUTS\1033\FORM.JS as well.