When you sort the DICT of a file, the file DICT.DICT is used as the dictionary, so it sounds like you have a malformed item in DICT.DICT.
Use the SEARCH command to look for FIELD.ASSOC in DICT.DICT. For example on my instance:
USER:SEARCH DICT.DICT *
1 Items selected to list #0
0002 @ID FMT "17L" CODE KEY.CODE CONV FORMAT NAME FMT "7L" SM ASSOC ID.SUP BY NAME
so in my DICT.DICT I don't have anything with FIELD.ASSOC_ in it, but the "@" item--which is used for a default list when no attributes are referenced--has a similar name "ASSOC". Maybe your "@" entry was overwritten by one from an imported account? Also use search to look at DICT TEST because even though it isn't used by the default list, CMQL might be parsing it. If you don't find anything in DICT.DICT or DICT TEST, try looking in VOC and DICT VOC as well.