To successfully export a Spine 4.2 project for use with the Unity runtime 4.1, you need to ensure that the versions of the Spine editor and the spine-unity runtime match. The JSON or binary files exported from Spine 4.2 are not compatible with the spine-unity 4.1 runtime. You should re-export your project using Spine 4.1 to ensure compatibility. If you have already created your project in Spine 4.2, you will need to open it in Spine 4.1 and export it from there. This process may involve some data loss if features used in 4.2 are not available in 4.1. Additionally, using the Spine JsonRollback tool to downgrade from 4.2 to 3.8 is not recommended for runtime use, as it is primarily for downgrading projects for use in older Spine editor versions.

Lusiver7 Could you please show us what is written in the log that can be opened via the Open Log button when an error occurs when importing a JSON file into Spine 4.1? Also, if possible, we would appreciate it if you could send us via email the JSON file where the import failed: contact@esotericsoftware.com
Please include the URL of this forum thread in the email so we know the context. Then we can take a look at what's wrong.

Lusiver7 Thank you for emailing us the text of the error log! According to the log, it seems that the animation "01_Idle_Breaker" has an inheritance timeline that cannot be converted to the 4.1 format, hence the error. Changing bone inheritance options in animations has been possible since 4.2, so this is not available in 4.1. Please modify the animation to work correctly without the Inherit key using Spine 4.2 first, and then try downgrading to 4.1.