I'm a fairly new user to Oxwall and have version 1.8, and all my plugins (except User Credits now) are up to date. It seemed strange to us as well, but the problems we were having are fixed now after having rolled back to the previous User Credits version.
I thought users at first simply had been blocked or had their privacy settings too narrow, but after several complaints over a few days, we looked into it more. My admin and I experienced this with our test accounts also. Even with all privacy settings set to "everyone" no new friends were added to the chat contact list, so chats couldn't be started with them (this didn't affect mails, only chats). The only change to the site in the previous few days had been upgrading the User Credits plugin, the chat contact list was still behaving as expected on our test site where we had not yet upgraded the User Credits plugin, so we decided to roll it back and see if that changed anything. It did.
Lost contacts were not recovered, but people can cancel and renew friendships to get those people to appear in their contact lists again now that we went back to the previous User Credits version.