Is it allowed to upload videos with a resolution of 1280x768 / 5:3

You could try submitting it but I would not be surprised if it's rejected for 'Non-compliance.'  Minimum size on both sides is 1080px.  768 is too small.

This is from Stock's Video Submission Guidelines.  Follow these & check back periodically.  Guidelines are subject to change.

https://helpx.adobe.com/stock/contributor/help/video-requirements.html

Horizontal Resolution 

• Upload source content in one of the following resolutions:  

1. 1920x1080 pixels 
2. 2048x1080 pixels 
3. 3840x2160 pixels 
4. 4096x2160 pixels 
5. 4096x2304 pixels 

• Upload content in its native frame size. Don’t down-convert or up-convert from the original camera resolution unless the source resolution is greater than 4K. 
• We accept video clips that are 1920 pixels wide up to full DCI 4K at 4096 pixels wide. 
• Standard sizes are 1920x1080 (HD), 2048x1080 (2K), 3840x2160 (UHD), 4096x2160 (4K), and 4096x2304 (4K-16x9). However, we also accept most wide screen aspect ratios such as 2.39:1 which can result in lower height pixel counts. 

Vertical Resolution

• Upload source content at one of the following resolutions:

   1. 2304x4096

   2. 2160x4096

   3. 2160x3840   

   4. 1080x1920

• Shoot vertical video natively and upload video in the clip’s 9:16 aspect ratio.  Do not convert the video to 16:9 aspect ratio and upload horizontally. 
• For vertical video, we recommend a duration of 10 to 30 seconds for each clip, although the minimum length is 5 seconds and maximum length is 60 seconds. 
• Please include the keywords “vertical” and “video” in your titles and keywords. 

Square Resolution

• Upload source content at one of the following resolutions:

      1. 1080x1080

      2. 2160x2160

• Upload video in the 1:1 aspect ratio.

Hope that answers your question.