Following are the steps to reproduce this issue:
1. Open a system, static lookup or create a user-defined static lookup with an attribute on the Project Object (or any object accessible in Modern UX)
2. Create a value in it with Name as "Test" and ID as "test/test"
3. Navigate to the Modern UX Projects list page
4. Pick "Project Type" filter
Expected Results: It is possible to pick values from which to filter the Project List
Actual Results: The following error gets thrown in the UI: 'API-1021 : Could not construct response. Unsupported character(s) found in the path component.'
This issue is caused due to a defect - DE48189.
The defect has been fixed in version 15.9.0.
Accompanied by the following error in the app-ca.log (Note the lookup ID from step 2 being logged here):
ERROR 2019-03-12 08:57:28,863 [http-nio-80-exec-75] ppm.rest (clarity:admin:5194048__7470BBC6-7899-40D2-8BCA-9E00C6AC0A9A:PPM_REST_API) JSONResponseBuilder :: Found path sensitive chars while appending path component[http://generichostname/ppm/rest/v1/lookups/OBJ_IDEA_PROJECT_TYPE/lookupValues] to url[test/test]