Not familiar with Arduido tools.
Are there any LED to indicates capsense button status? If LED blink can follow capsense button's ON/OFF status, this issue must be caused by host firmware or I2C bus
How about contacting attiny85/Arduino's tech support?
Thanks a lot for all your answers! I found out what the problem was:
Using the tinyWireM library for attiny85, when receiving a 0 value (e.g when polling the buttons status and no one is triggered), the I2C break. I don't know yet why, but will investigate.
A quick fix is to re-init the I2C ( tinyWireM.begin() ) just after a buttons reading, this fix the problem.
Hope it will be usefull for someone else one day!
Thanks you anyway for your time and for the other answers given ( I didn't thanks you before, sorry, was in a rush)