Exporting with a png sequence overlaid

hello there.

i'm trying export 2 minute clip consists of mp4s , overlaid png sequence exported animate. when attempt export the, usual, 'unknown error' @ time first transparency appears in whole sequence. when take out png sequence, can export fine. so:

i have tried exporting animated elements animate .mov (with alpha channel). when import resulting .mov file pp, insists audio file, despite playing - video - in vlc.

i have looked @ various answers here saying track should targeted video, or source patching needs active video track i'm attempting drop .mov into. none of has made difference. in sequence 'library' (or whatever window called lists imported available elements in project) .mov file listed audio elements , displays soundwave icon - no video icon included.

so:

my question depends on approach work:

 

how can use png sequence, transparency without failing @ export?

or:

how can import .mov file include video transparency? exporting-from-animate issue or premiere import setting?

 

any help, appreciated.

thanks.

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