there is a maximum pathlength, but you shouldn't hit that unless you're creating some outlandishly large paths. The maximum length should be set to 255 characters. Earlier versions of Ariadne had this set to 128, which you can hit with relatively sane paths. It might be that you haven't properly upgraded Ariadne. In the www/install/upgrade/ directory is a script to automatically run all upgrades that you may have missed. It should also make sure that the maximum path length is set to 255 characters.
If you need more than that, you should reconsider your site structure, but with a modern MySQL or mariadb database, and possibly with PostgreSQL as well, you should be able to configure your database to accept longer paths. The table to alter is the ariadne_nodes table, where you need to increase the length of the path and parent columns.
alter table ariadne_nodes modify path varchar(1024), modify parent varchar(1024);
This is from memory, so you should check your database manual for the correct syntax :)
However, by making the path and parent columns bigger, the path index will need more memory and become slower, so my original advice stands, keep path lengths below 255, smaller is always better.
PS. if you aren't sure if this is the problem, check the contents of the database. The structure isn't hard to understand, the full path is stored in the table ariadne_nodes in the column path. If it is truncated, check the definition of the column, if it is set to varchar(128) you'll need to run the upgrade script.