Hi Harri,
What kind of authentication are you doing for these users? MAB, Dot1x? Also is this issue seen with all devices, or just a few ( i.e. same type, same vendor...)?
If this is self-registration for guest users, there is a known issue with using Custom Guest Portal. The defect details are given below :
https://tools.cisco.com/bugsearch/bug/CSCui77336/?reffering_site=dumpcr
Therefore if you are using the custom portal, can you instead try with a default portal?
Thanks,
Aastha