The server started correctly and there is no error. But if my friends and me want to join, we always get the same error:

Please help

btw server version and client version are 2.5.7

No, actually the server I believe is 2.5.7, and your clients are 2.5.9 as that is the error people who updated the pack would get when joining the server before it was updated.

Well, first I used client 2.5.9 and then switched to 2.5.7. I got the modpack from CurseForge; there I changed the version to 2.5.7.

Update: While writing the comment I saw that the version was 2.5.9 ... I thought the whole time it was 2.5.7. That was a problem with CurseForge...

Problem solved. thank you!