Cannot save skins to local library of skins

I can no longer use the "Skins" interface to save skins. Without being able to save them, I can't apply a new skin from the launcher. Even if I apply the new skins on the website, clicking "Add to library" is useless and gives and error message of

To make "Skins" work in the first place, I had to softlink to Chrome's "swiftshader" like so:

$ ls -l /opt/minecraft-launcher
total 182992
-rw-r--r-- 1 root root 621568 Jul 14 02:42 cef_100_percent.pak
-rw-r--r-- 1 root root 730195 Jul 14 02:42 cef_200_percent.pak
-rw-r--r-- 1 root root 3381507 Jul 14 02:42 cef_extensions.pak
-rw-r--r-- 1 root root 4115814 Jul 14 02:42 cef.pak
-rwxr-xr-x 1 root root 26592 Jul 14 02:42 chrome-sandbox
-rw-r--r-- 1 root root 10196592 Jul 14 02:42 icudtl.dat
-rwxr-xr-x 1 root root 125944304 Jul 14 02:42 libcef.so
-rw-r--r-- 1 root root 8584 Jul 14 02:42 libgconf-2.so.4
-rw-r--r-- 1 root root 38505944 Jul 14 02:42 liblauncher.so
drwxr-xr-x 2 root root 4096 Jul 29 22:33 locales
-rwxr-xr-x 1 root root 125192 Jul 14 02:42 minecraft-launcher
-rwxr-xr-x 1 root root 218275 Jul 14 02:42 natives_blob.bin
-rwxr-xr-x 1 root root 1530292 Jul 14 02:42 snapshot_blob.bin
lrwxrwxrwx 1 root root 30 Jul 29 22:40 swiftshader -> /opt/google/chrome/swiftshader
-rwxr-xr-x 1 root root 1948168 Jul 14 02:42 v8_context_snapshot.bin
$

So, if that needs to point somewhere else, let me know. I created the softlink based on error messages and some hints I found online. I don't recall the details...

This worked in a previous version of the Launcher, though I'm unsure which versions exactly...