TYPO3 Core

I'm trying to update a website from Typo3 4.5.x to 6.1.1

In the install tool, I get the following warnings while doing the upgrade:

Upgrade Wizard:
Step 3 - Perform updates

Migrate all file relations from tt_content.image and pages.media
Update successful!

File 'uploads/pics/81d8165a33_23_67_font.png' does not exist. Referencing field: tt_content.67.image. The reference was not migrated.

File 'uploads/pics/0fd3e52a0e_23_68_font.png' does not exist. Referencing field: tt_content.68.image. The reference was not migrated.

File 'uploads/pics/c6601359f6_23_69_font.png' does not exist. Referencing field: tt_content.69.image. The reference was not migrated.

File 'uploads/pics/5cd917c4fe_23_70_font.png' does not exist. Referencing field: tt_content.70.image. The reference was not migrated.

File 'uploads/pics/6c55a918b2_23_71_font.png' does not exist. Referencing field: tt_content.71.image. The reference was not migrated.

File 'uploads/pics/5c7a80fb3c_23_72_font.png' does not exist. Referencing field: tt_content.72.image. The reference was not migrated.

File 'uploads/pics/4d4bb1490a_23_73_font.png' does not exist. Referencing field: tt_content.73.image. The reference was not migrated.

File 'uploads/pics/9129a09df1_23_74_font.png' does not exist. Referencing field: tt_content.74.image. The reference was not migrated.

(and some more, similar lines)

The result is that some pictures are missing, some are randomly(?) replaced and some appear 3 times instead of once.

If you want to see it yourself - www.suedpfalz-verlag.de is the working version (still 4.2, but updating to 4.5 works without problems), and spverlag.cboltz.de is the broken version after the update.

If you want me to test something - I can easily restore the 4.5 version from backup and do another update.