[ERR 15:36:12.446] Texture 'SSTU/Assets/SC-TANKS-SLS-NRM' not found!

[LOG 15:36:12.455] Load(Model): SSTU/Assets/SC-ADPT-SOYUZ2
[ERR 15:36:12.459] Texture 'SSTU/Assets/SC-TANKS-SLS-DIFF' not found!

[ERR 15:36:12.459] Texture 'SSTU/Assets/SC-TANKS-SLS-NRM' not found!

[LOG 15:36:12.467] Load(Model): SSTU/Assets/SC-ADPT-SOYUZ3
[ERR 15:36:12.470] Texture 'SSTU/Assets/SC-TANKS-SLS-DIFF' not found!

[ERR 15:36:12.470] Texture 'SSTU/Assets/SC-TANKS-SLS-NRM' not found!

[LOG 15:36:12.477] Load(Model): SSTU/Assets/SC-ADPT-SOYUZ4
[ERR 15:36:12.482] Texture 'SSTU/Assets/SC-TANKS-SLS-DIFF' not found!

[ERR 15:36:12.482] Texture 'SSTU/Assets/SC-TANKS-SLS-NRM' not found!

I have checked them, they are just not present in the .zip in the release.
There is just a load of such messages through the .log when SSTU loads the data.

Well, that explains a whole mess of errors present for many other mods.

Thanks for this explanation.

This is caused by the fact that the texture names are embedded in the model (.mu) files by the KSP part exporter.

I have long since changed the textures used by those, but the original names are still embedded in the files (and cannot be manually fixed).

In short -- not an error, you can safely ignore those messages.