When I try to render proxy clips, I always get errors (at least after some clips). Error message is:

[matroska,webm @ 0x7fc3ead039c0] Duplicate element
[matroska,webm @ 0x7fc3ead039c0] Duplicate element
[matroska,webm @ 0x7fc3ead039c0] Duplicate element
[matroska,webm @ 0x7fc3ead039c0] Duplicate element
[matroska,webm @ 0x7fc3ead3cfc0] Duplicate element
[matroska,webm @ 0x7fc3ead3cfc0] Duplicate element
[matroska,webm @ 0x7fc3ead3cfc0] Duplicate element
[matroska,webm @ 0x7fc3ead3cfc0] Duplicate element
[matroska,webm @ 0x7fc3ead3cfc0] Duplicate element

Tested an checked with all AppImage builds sind 19.12.x (incl. 20.08).