A neat one this.
Users complained that they suddenly got status 75 (can't open file) or status 74 (can't open filelist.cfg) on character mode programs; however windows programs all worked just fine. Having verified that all files were indeed in the right places I dug in a bit further.

The timing coincided with me creating a new workspace pointing at existing data folders. For historical reasons (a brief episode of multi-tenancy) our production filelist is not in the first folder of the data path but in the second. So a normal .ws includes the lines ...
DataPath=\\KF6W\KF6W_data\working\kirk1;\\KF6W\KF6 W_data\kirk1;\\KF6W\KF6W_data\common

Seems that the new workspace creation doesn't check the full data path for an existing filelist and just bungs an empty one in the first folder. However it equally doesn't apply any intelligence to what it puts in the .ws for the filelist path. It just tacks 'filelist.cfg' onto the full pathlist, so ....
FileList=\\KF6W\KF6W_data\working\kirk1;\\KF6W\KF6 W_data\kirk1;\\KF6W\KF6W_data\common\Filelist.cfg

The windows programs sharing the data continued to work because there is an explicit path to the correct filelist.cfg in the .ws; whereas the charmode programs crashed because they open the first filelist.cfg they come to, which is the new, empty, one in the \\KF6W\KF6W_data\working\kirk1 folder.

My techs now know the issue and it won't bite us again, but I think that just adding 'filelist.cfg' to the end of the data path is a little simplistic and maybe the create new workspace wizard should demand an explicit location.