I have the same issue.
In my case, trying to create a run training of type Time. An easy example to reproduce the issue:
1) Create a run session
2) Delete the warm-up and the cooling lap
3) Edit the run lap and make it of type "Time"
Is not possible to register 'cause a failure system....
Lol.... I was trying to upload the image in a web-images and make an Insert/Images..... :-P
An easy example to reproduce the issue:
1) Create a run session
2) Delete the warm-up and the cooling lap
3) Edit the run lap and make it of type "Time"
Then, trying to save, is not possible to register 'cause a failure system....
Its not the only combination where you can achieve the issue. The problem is only generated in the web app, 'cause if I try to do it by the clock I dont have any problem.