I upgraded my site from 2.1 to 2.6 and I get this error when trying to go to feildtypes:
Unable to load requested field type file: ft.rel.php. Confirm the fieldtype file is located in the expressionengine/third_party/ directory
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I posted the below comment here: Unable to load requested field type file: ft.rel.php
I had the same problem only it was "ft.matrix_url_title.php"
I am on 2.6.1 and Matrix 2.5.5
The weird thing was that it was ok on my local machine but not on my dev server.
Turns out that the db table exp_fieldtypes on the dev server had the field type name ("name" in the db column) as "matrix_url_title" whereas the local db had "matrix". Once I changed the dev server db to matrix it worked. So there was obviously a conflict when upgrading either to 2.6.1 (from 2.4.0) or to Matrix 2.5.5.