The Issue

Deltares currently has an old SVN server that is <= 1.4. Therefore, some components of TortoiseSVN client >= 1.6 don't work. Especially merging with mergeinfo is not supported. SVN will show you the following error when trying to merge your branch back into the trunk:

Command: Auto-merge https://repos.deltares.nl/repos/delft-tools/branches/delta-shell-spatial-editor into D:\kraus\svn\delta-shell\trunk
Error: The requested feature is not supported by
Error: '/repos/delft-tools/!svn/bc/27691/trunk/delta-shell'
Error: Additional errors:
Error: Querying mergeinfo requires version 3 of the FSFS filesystem schema; filesystem
Error: '/part0/repositories/delft-tools/db' uses only version 2
Completed!

You cannot use the option for merging the branch back into the trunk: 'Merge a specific range of revisions' and then set it to 'all revisions'.

What still works (trunk to branch)

Making your branch up-to-date by merging the trunk into the branch works fine. You choose to 'Merge a specific range of revisions' and you select the 'specific range' that is not yet included in the branch.

The Solution (branch to trunk)

Use the old merge option. To merge your branch back into the trunk use the old option 'Merge two different trees'.
Then set the following options in TortoiseSVN:

From: 
URL: the trunk
Revision: The last revision that was merged into your branch. This can be the HEAD revision if your branch is completely up-to-date.

To:
URL: the branch you want to merge into the trunk
Head Revision