An Error occurred try again later when trying to modify subscription

I found the answer after reading a myriad of articles.

The problem for me is that the RSExecRole was created without any grants added to it.

So, I found this very helpful article that tells you how the RSExecRole should be setup:

https://docs.microsoft.com/en-us/sql/reporting-services/security/create-the-rsexecrole?view=sql-server-2017

The bottom line is that you need to run this script.

USE master;
GO
IF NOT EXISTS (SELECT 1 FROM sys.database_principals WHERE [type] = 'R' AND [name] = 'RSExecRole') BEGIN
CREATE ROLE [RSExecRole];
END
GRANT EXECUTE ON dbo.xp_sqlagent_enum_jobs TO [RSExecRole];
GRANT EXECUTE ON dbo.xp_sqlagent_is_starting TO [RSExecRole];
GRANT EXECUTE ON dbo.xp_sqlagent_notify TO [RSExecRole];
GO
USE msdb;
GO
IF NOT EXISTS (SELECT 1 FROM sys.database_principals WHERE [type] = 'R' AND [name] = 'RSExecRole') BEGIN
CREATE ROLE [RSExecRole];
END
GRANT EXECUTE ON dbo.sp_add_category TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_add_job TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_add_jobschedule TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_add_jobserver TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_add_jobstep TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_delete_job TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_help_category TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_help_job TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_help_jobschedule TO [RSExecRole];
GRANT EXECUTE ON dbo.sp_verify_job_identifiers TO [RSExecRole];
GRANT SELECT ON dbo.syscategories TO [RSExecRole];
GRANT SELECT ON dbo.sysjobs TO [RSExecRole];
GO

After that I could edit subscriptions and users could create and edit them.

Hope this helps someone.