Session [null] not available and is not among the last 1000 terminated sessions

[FlexiB]

When I try to execute a test with this plateform and only with Firefox I have this issue :

Session [null] not available and is not among the last 1000 terminated sessions

Plateform :

• Node with selenium-3.5.2

• Grid with selenium -3.5.2

• Selenograph 1.2 (with GridRouter)

This is OK when I execute on Chrome, Chome Headless, IE (7, 9, 10, 11).

Maybe someone has an idea ?