Okay, the messages are now in the appropriate topics.
The relevant errors in the log are:
Type of error: Undefined
Notice: Undefined index: new_topic_subject
https://www.elkarte.net/community/index.php?action=splittopics;sa=selectTopics;subname=Footer+erg%C3%A4nzen;topic=5395.0;start2=0;move=down;msg=38526;xml
File: /.../sources/controllers/SplitTopics.controller.php
Line: 333
Type of error: Undefined
Notice: Undefined index: new_topic_subject
File: /.../sources/controllers/SplitTopics.controller.php
Type of error: Undefined
Notice: Undefined index: new_topic_subject
https://www.elkarte.net/community/index.php?action=splittopics;sa=selectTopics;subname=Footer+erg%C3%A4nzen;topic=5395.0;start2=0;move=down;msg=38526;xml
File: /.../sources/controllers/SplitTopics.controller.php
Line: 410
Type of error: Undefined
Notice: Undefined index: move_to_board
https://www.elkarte.net/community/index.php?action=splittopics;sa=splitSelection;board=19.0
File: /.../sources/controllers/SplitTopics.controller.php
Line: 266
Type of error: Undefined
Notice: Undefined index: new_topic_subject
https://www.elkarte.net/community/index.php?action=splittopics;sa=splitSelection;board=19.0
File: /.../sources/controllers/SplitTopics.controller.php
Line: 269
Type of error: Undefined
Notice: Undefined index: messageRedirect
https://www.elkarte.net/community/index.php?action=splittopics;sa=splitSelection;board=19.0
File: /.../sources/controllers/SplitTopics.controller.php
Line: 277
Then, but this is due to the fact the topic doesn't have messages, so it could be overlooked because is a situation that in theory should not happen:
Type of error: Critical
Database error, given array of integer values is empty. (messages)
Function: followupTopics
https://www.elkarte.net/community/index.php?topic=5403.0
File: /.../sources/subs/FollowUps.subs.php
Line: 37