"Error occurred while connecting to termbase" for guest user in Multiterm Online

We have Multiterm Online component purchased and installed and the 'guest' user for people with no GroupShare content. Previously, the 'guest' user worked fine, but recently we've been reported that, after logging in with guest user and trying to open any termbase, the following error message appears: "Error occurred while connecting to termbase "<termbase name>". (Detail: You do not have access to the following termbase: '<termbase name>'. Contact your MultiTerm administrator if you need to change your user rights.)"

That happens for any server-based termbase. When logging in as other users, everything works correctly.

What we have checked: Multiterm Guest User is present among our GroupShare users with the Multiterm Guest role. The role is unchanged (we cannot actually edit it).

Every termbase has been checked in desktop Multiterm in the Administrator mode. In the Restriction section the guest user is included in the Public restriction. We tried to add Multiterm Guest role to the restriction as well as a GroupShare role, but that didn't help.

Any idea what could go wrong here?