I thought it might be that too, especially since the cases in config.txt don't match that of the script file;
script "PortalTunnel"
class "PortalTunnel"
script filename = portaltunnel.gs
However, when I changed the 2 config.txt entries to all-lowercase, exactly matching the script name, the 'link' error was still there.
I reverted to original, which unsurprisingly eliminated the error report. I then Opened for Edit, still no error (which was surprising). I was even able to make the following edits and re-commit without causing the error to return;
- removed the messy looking foreign language descriptions and string-table tags.
- removed the Type tag.
- converted category-era-x tags to one category-era tag with "1800s" as the only value.
- converted category-region-xx tags to one category-region with "UK" as the only value.
- added a category-keyword tag with some keywords.
- changed username from "Re-rail Portal Basic" to "Portal re-rail basic".
Note that the script/class tags, with their mixed lower/uppercase letters were left untouched, and as I said, the fault did not reappear.
I am not sure what to conclude from all this, apart from thinking that I must have previously made some edit other than the ones mentioned above which triggered the fault. And it seems that you can actually get away with mismatching cases between the config and script filename.