yes because some users have this error and some user doesn't have this error and i have tried it from different browsers it is not a browser issue
here is a closest defect;
note: latest version of cucm 10.5.2 and onwards donot show this defect in lab; however the below workaround might still help.
try this workaround;
Under System > Enterprise Parameters, change the following settings:
Show Phone Display Locale > False
Default User Locale > English United State
Then the User Locale can be applied for the phones through Common Device Configuration under Device > Device Settings on the Cisco Unified CM Administration
if possible try this in a brief maintenance window.
- rate if this helps.