I'm running Linux Mint 12 "Lisa," with the MATE desktop environment if that matters. Which makes this DigiKam v2.1.1 on KDE Development Platform 4.7.4

Everything in DigiKam seems to be working out so far, I like it better than Picasa--except I can't get it to create any new tags. Every time I try, say, to make the tag "tree," it says

An error occurred during tag creation:
Tag Path: /tree
Error: Failed to add tag to database

...And that's it. What do I need to do?

seemes to me like a simple "not given" ore "wrong given" pathing problem
try following
open up synaptic
by klicking on "adjustment" in this tool, there is a point that will give you information about, where all the relevant files of a program / tool are installed
if you added this to your "view"
start your favorite commander
for example mc (midnight commander)
move to the relevant file & try to edit
don't forget to make a save-copy of the file you're edeting
regards

I'm sorry, what? Click on the "adjustment" tool in Synaptic? I have no idea what you're talking about, and I can't find anything labeled "adjustment" in the Synaptic context menu or toolbar menus.

Well, I finally tried just deleting and rebuilding the database file, and that seemed to do the trick. This wouldn't be a very good solution to anyone who hadn't set the metadata to write to file because they would lose any tags they had... but if it's a problem of a corrupt database file, then maybe they would have lost the tags anyway.

l.t. wrote:...
by klicking on "adjustment" in this tool,...

even look for something like "adjustment"
mine is in german, dont know yours
if you don't find this settings thing...
try open a terminal & surch by "whereis filename"
maybe this will help
than go on with the steps I told before