Cannot install Anek Tamil on Ubuntu

[jheinecke]

I tried to install Anek Tamil on Ubuntu, but even though it appears in .local/share/fonts/Unknown Vendor/TrueType/Anek Tamil it is not shown by fc-list (all the other Anek fonts install well and work. When I tried to recreate the ttf file with gftools builder AnekTamil/builder.yaml I get the following error:

...
INFO:fontTools.varLib:Merging OpenType Layout tables
fontmake: Error: In 'Masters/AnekTamil.designspace': Generating fonts from Designspace failed: 

Couldn't merge the fonts, because some values were different, but should have
been the same. This happened while performing the following operation:
GPOS.table.FeatureList.FeatureRecord[0].Feature.LookupListIndex[0]

The problem is likely to be in Anek Tamil Condensed Thin:
Expected to see [0]==5, instead saw 4

ninja: build stopped: subcommand failed.
Removed file: /home/myhome/fonts/Anek/sources/AnekTamil/build.ninja

Here I'm completely stuck. Is this a problem of the Anek Tamil font (file) or due to me using python 3.8 ?