To repro this bug, you need to change your system settings so that the decimal separator is , instead of .
Suggestion: the Swedish settings should repro this bug. Here is how to change the region settings on windows: https://support.office.com/en-us/article/change-the-windows-regional-settings-to-modify-the-appearance-of-some-data-types-edf41006-f6e2-4360-bc1b-30e9e8a54989
--> Change the region settings to Swedish

1) Add the CustomClip files provided in this bug report into your project.
2) Create a new timeline.
3) Add a new Playable Track.
4) Add a CustomRecordableClip to this track
5) Click on the clip, open the inline curves.
6) In the inspector, change the Volume value to 20.1 (a number with decimals is important)
7) Right click on the Volume label, click on Add Key

Result: No key was added, an error message shows up in the console.
Expected: Should add a key to the clip.