Actually I had the same error where some files were parsed correctly by the engine and others did not.
After a quick skim through the source code, I managed to fix the issue. Before explaining the solution I would like to point out that I have been using a Light Simulation engine that skipped some parameters during parsing. Now according to the UE version 4.26 the following parameters matter the most for IES version (IESNA:LM-63-2002):
<#lamps>< lumens/lamp>,< multiplier> <# vertical angles><# horizontal angles>< photometric type>< units type>< width>< length>< height>
The problem in my case was the <#lamps> parameters was 0 and according to the check in image (InvalidLampCount.png) it will raise an error eventually.
Now for where to edit this part check [this link][1] as a reference for the description of file content. I have also attached an image depicting quickly where to look at the mentioned parameters above in a sample.