This may be the result of a file that was downloaded improperly (truncated or damaged).
Please perform the following steps to address this:
Bring up your debug.log file (located in c:\mythic\camelot) in Wordpad. Scroll to the very bottom of the file. Usually, there will be a file listed at the bottom with the text ERROR next to it. For example:
Error Loading NIF #151 (effects/mid_darkblast_hit.nif)!
Go through the log and DELETE all lines with the word error attached to them (you can jump to these lines by going to EDIT -> FIND and typing the word "error" in the "find" window). As you delete lines, locate the files listed in those lines from your c:\mythic\camelot directory and delete them as well. Bear in mind that some files will be in separate folders within this directory. For example, the listing "effects/mid_darkblast_hit.nif" refers to the file named "mid_darkblast_hit.nif" inside of the "effects" subfolder within c:\mythic\camelot.
The newly-deleted files will be redownloaded when you reconnect to DAoC.