When i disable all interrupts with EA=0 inside the I2C-ISE, and a change of the SCL line occures, before i reenabled the interrupts with EA=1, can i loose this event???

If a SCL change is supposed to trigger the IIC ISR, you will not detect that event before the processing of the IIC ISR is completed (and one instruction of main has been executed).
I am not familiar with the LPC, but normally and interrupt triggered while the interrupts are disabled will interrupt when they are enabled again.
One trap someone fell in (it to me a while to see it) was that the ISR reset the interrupt (not interrupt enable) bit and thus a second interrupt coming before that was lost.

If you must manually reset the interrupt trigger bit for the IIC ISR, make sure that is the first thing you do.

Have fun,

Erik