I had exactly the same thing happen. On mine, the default particle filter was attached when I got the machine, and the hose had a cut out notch (not sure if this is factory default or something the reseller did to differentiate the hoses), when I tried to add the water softener it split, and I had to cut a clean edge to fix it. I gave up on the softener, gonna use 3WW + distilled for the time being until I can make sure my filtered tap water is within range to use in the machine.
Anyway, the hose does seem awfully small diameter to fit the water softener - I could PROBABLY squeeze it into the hose with a flat edge, definitely not with a notch. Either way it’s too tight - they should really use the same diameter insert for both the water softener and particle filter, and use an appropriate guage hose that fits both.