With the help of some members on this forum I was able to retrieve my bitcoins, simply by entering the keys manually in Electrum. I can now send and receive bitcoins. I cashed in some of it and will keep using Electrum as a kind of savings account.
While I was entering the keys in Electrum I got the warning that imported keys could not be recovered using the seed.

After the fork of August 1 I also have a nice amount of BitcoinCash BCH. Is it possible to use a program like Jaxx or Electron Cash on another computer so I can use it for daily transactions? I don' t want to make a stupid mistake and lose all my bitcoin in the process.

I found a video on Youtube explaining how you could use the seed of Electrum and import it into Electron Cash to claim your BCH. But in my case that will not work: the key was not generated by Electrum but by Multibit years ago.

Any ideas/suggestions on how to solve this problem? I would appreciate it very much. Maybe there is another way I can access my BCH.