This configuration of the organisation raises an bad request by opening the portal or running a process out of the studio. This depends on the '\' in the username, which isn't correct encoded for the browser. (see attached screenshots)
Maybe this error applys BBPMC-501, too.
This was no problem in version 7.4.3
Bonita Studio 7.5.2
Browser: Mozilla Firefox or Chrome
Use usernames without special characters.