Encoding error

I tried to encode a movie with subs, but get an encoding error on three different PCs. Everything else worked fine (using it for month now).

 

Also I have two other questions:

-When I tried it last time, the external .ass subtitle file has not been recognized. I always have to remux the file with MKV ToolNix to replace the old subtitle file with my file. Is it only an error from my side or can you help me there?

-Is it possible to reencode the video with hardsubs without the need to set the video quality? (currently I use video quality 21 but I only want to hardsub it, maybe with H264 instead of the source but thats it.) Also it seems not to be possible to select video copy and hardsub subtitles at the same time.

 

Log:

[info] ShanaEncoder v5.2.2.0
[info] FFmpeg v2021.03.15.185
(removed Hardware since I tried it on three devices)

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 >> Encoding start time: 30.07.2021 16:24:40

 * Input filename: >removed<.mkv

Stream mapping:
  Stream #0:0 -> #0:0 (hevc (native) -> h264 (libx264))
  Stream #0:1 -> #0:1 (copy)
[Parsed_subtitles_0 @ 0000000008fe0040] Shaper: FriBidi 1.0.7 (SIMPLE)
[libx264 @ 0000000008fa06c0] using SAR=1/1
[libx264 @ 0000000008fa06c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 Cache64
[libx264 @ 0000000008fa06c0] profile Constrained Baseline, level 4.0, 4:2:0, 8-bit
[libx264 @ 0000000008fa06c0] 264 - core 161 r3048 b86ae3c - H.264/MPEG-4 AVC codec - Copyleft 2003-2021 - http://www.videolan.org/x264.html - options: cabac=0 ref=1 deblock=1:0:0 analyse=0x1:0x111 me=hex subme=2 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=0 threads=3 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=250 keyint_min=23 scenecut=40 intra_refresh=0 rc_lookahead=10 rc=crf mbtree=1 crf=21.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[mp4 @ 0000000008fdcec0] track 1: codec frame size is not set
[mp4 @ 0000000008fdcec0] flac in MP4 support is experimental, add '-strict -2' if you want to use it.
Could not write header for output file #0 (incorrect codec parameters ?): Experimental feature
Error initializing output stream 0:0 -- 
Conversion failed!

 * An encoding error has occurred.

 >> Speed 0x Elapsed time 00:00:00

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