I had a crash of my system.
I have re-installed the newest version of Electrum (3.3.4) and i did the following:
File
New/Restore
Fil in "default_wallet"
Next
Standard wallet
Next
I already have a seed
Next
Options
Extend this seed with custom words
OK
I fill in my seed + extend custom word
Type 12 words -> click Next -> type your custom words.