Hey martimiz,
Thanks for taking the time to investigate this further, and trial it your side.
I double checked for any duplicates in phpMyAdmin, by running these three SQL queries:
SELECT * FROM `SiteTree` WHERE `URLSegment` = 'project'
SELECT * FROM `SiteTree_live` WHERE `URLSegment` = 'project'
SELECT * FROM `SiteTree_versions` WHERE `URLSegment` = 'project'
No results for returned for the above, and tried again via the CMS and '-2' was still appended.
So I decided to just change it directly in the phpMyAdmin for SiteTree, SiteTree_live and SiteTree_versions. I then tried to navigate to the page in the front-end, and very interestingly I get a white screen with this error:
Action 'project' isn't allowed on this handler.
Do this shed anymore light on the possible cause? Is 'project' not allowed as a URLSegment in SS for some reason?