wc_db.c line 10235: assertion failed

I want to set an external reference to a relative project path, which does not contain spaces (googling the assertion gave a hint that spaces can be involved into creating this assertion).
I can enter the path but once I press the context menu entry "Fetch HEAD revision and adjust to it",
I get the assertion with the hint (svn_dirent_is_absolute(local_abspath)).
The local path is 'XXX', the referenced project is located at '../../../reference_project/trunk/my_src/XXX' and should exist,
as a second parallel project which is setup in similar fashion and directory structure can actually reference the same sub-directory.

My Tortoise svn is
TortoiseSVN 1.9.3, Build 27166 - 64 Bit -dev, 2016/02/08 07:58:04
on Windows 7.
Received on 2016-03-21 20:51:36 CET