WF-7620 paper jam message without paper jam

[maximilian59]

I got a WF-7620 to check. It does the nozzle check with some bad nozzles at black, printing with quality set to Standard on plain paper. But if I choose a different paper or higher quality setting, the problem starts.
The paper is feeded just over the sensor and is visible just after the rolls before the printhead. There it stops, the it is needed just a bit under the printhead and stops there with the error message.
The printer shows a lot of errors with WIicreset Tool:

Model Name WF-7620 Series
Status Printer is ready to print
Firmware ver. AD24J4 24/04/2019
Serial Number
Print Head ID 03E3F4833UVUTVTYXXS340%
Manual cleaning counter 86
Timer cleaning counter 0
Ink replacement cleaning counter 27
Total print pass counter 31856
Total print page counter - cassette 1616
Total print page counter - manual 0
Total print page counter - duplex 131
Total scan counter 117
Total scan counter (ADF) 346
First TI received time 06/02/2016
Ink replacement counter - black 0
Ink replacement counter - cyan 0
Ink replacement counter - magenta 0
Ink replacement counter - yellow 0
Maintenance box replacement counter 1
Last printer fatal error code 1 0xEA CR LOAD POSITION excess load error. CR encoder failure. CR motor failure. Carriage overload. Tooth skip or improper tension of the timing belt. Cable or FFC disconnection.
Last printer fatal error code 2 0xE5 CR PID speed fall error. CR encoder failure. Motor driver failure. Main board failure. Tooth skip or improper tension of the timing belt. Paper jam.
Last printer fatal error code 3 0x8D Factor other than printer device error. Scanner or ADF failure.
Last printer fatal error code 4 0xE3 CR PID reverse error. CR encoder failure. External force or vibration. Tooth skip or improper tension of the timing belt. Paper jam. Motor driver failure. Main board failure.
Last printer fatal error code 5 0xE3 CR PID reverse error. CR encoder failure. External force or vibration. Tooth skip or improper tension of the timing belt. Paper jam. Motor driver failure. Main board failure.
Last scanner fatal error code 1 0x36 Scanner/ADF paper jam error. Paper jam. Foreign object. Loading or ejecting paper out of the standard range. Using long paper.
Last scanner fatal error code 2 0x05 Scanner/ADF PID excess load. Encoder failure. Motor failure. Mechanism overload. Cable or FFC disconnection.
Last scanner fatal error code 3 0x10 Scanner/ADF home position detection error. CIS Module failure. Upper housing failure. Lower housing failure. Scanner motor failure. Insufficient grease. Foreign object. Cable disconnection. Main board failure.
Last scanner fatal error code 4 0x10 Scanner/ADF home position detection error. CIS Module failure. Upper housing failure. Lower housing failure. Scanner motor failure. Insufficient grease. Foreign object. Cable disconnection. Main board failure.
Last scanner fatal error code 5 0x05 Scanner/ADF PID excess load. Encoder failure. Motor failure. Mechanism overload. Cable or FFC disconnection.

The printer was used with a CISS. Is it worth to start repairing? I will do it by myself, but I don't know hoe I can resolve the problem. Is it electronically or mechanical. I have a service manual, but don't want to begin disassembly without knowing for what to look.
Any hints welcome.

Maximilian