I've had this same issue all year and nothing seems to fix it. I finally updated from Mybb version 1.8.15 today to 1.8.19 and updated all plugins. MentionMe is now version 3.2.7. MyAlerts is on 2.0.4 and apparently that's the most recent version.
The only error in my error log which might be relevant (I can't tell for sure but I don't think it is) is this:
<error>
<dateline>1521154791</dateline>
<script></script>
<line>0</line>
<type>20</type>
<friendly_type>MyBB SQL Error</friendly_type>
<message>SQL Error: 2006 - MySQL server has gone away
Query: SELECT title,template FROM mybb_templates WHERE title IN ('' ,'member_register' ,'member_register_hiddencaptcha' ,'member_register_coppa' ,'member_register_agreement_coppa' ,'member_register_agreement' ,'member_register_customfield' ,'member_register_requiredfields' ,'member_profile_findthreads' ,'member_loggedin_notice' ,'member_profile_away' ,'member_register_regimage' ,'member_register_regimage_recaptcha' ,'member_register_regimage_recaptcha_invisible' ,'member_register_regimage_nocaptcha' ,'post_captcha_hidden' ,'post_captcha' ,'post_captcha_recaptcha' ,'member_register_referrer' ,'member_profile_email' ,'member_profile_offline' ,'member_profile_reputation' ,'member_profile_warn' ,'member_profile_warninglevel' ,'member_profile_customfields_field' ,'member_profile_customfields' ,'member_profile_adminoptions' ,'member_profile' ,'member_profile_signature' ,'member_profile_avatar' ,'member_profile_groupimage' ,'member_profile_referrals' ,'member_profile_website' ,'member_profile_reputation_vote' ,'member_activate' ,'member_lostpw' ,'member_register_additionalfields' ,'member_profile_modoptions_manageuser' ,'member_profile_modoptions_editprofile' ,'member_profile_modoptions_banuser' ,'member_profile_modoptions_viewnotes' ,'member_profile_modoptions_editnotes' ,'member_profile_modoptions_purgespammer' ,'usercp_profile_profilefields_select_option' ,'usercp_profile_profilefields_multiselect' ,'usercp_profile_profilefields_select' ,'usercp_profile_profilefields_textarea' ,'usercp_profile_profilefields_radio' ,'member_viewnotes' ,'member_register_question' ,'member_register_question_refresh' ,'usercp_options_timezone' ,'usercp_options_timezone_option' ,'usercp_options_language_option' ,'member_profile_customfields_field_multi_item' ,'member_profile_customfields_field_multi' ,'member_profile_contact_fields_aim' ,'member_profile_contact_fields_google' ,'member_profile_contact_fields_icq' ,'member_profile_contact_fields_skype' ,'member_profile_contact_fields_yahoo' ,'member_profile_pm' ,'member_profile_contact_details' ,'member_profile_banned_remaining' ,'member_profile_addremove' ,'member_emailuser_guest' ,'member_register_day' ,'usercp_options_tppselect_option' ,'postbit_warninglevel_formatted' ,'member_profile_userstar' ,'member_profile_findposts' ,'usercp_options_tppselect' ,'usercp_options_pppselect' ,'member_resetpassword' ,'member_login' ,'member_profile_online' ,'usercp_options_pppselect_option' ,'postbit_reputation_formatted' ,'member_emailuser' ,'usercp_profile_profilefields_text' ,'member_profile_modoptions_ipaddress' ,'member_profile_modoptions' ,'member_profile_banned' ,'member_register_language' ,'member_resendactivation' ,'usercp_profile_profilefields_checkbox' ,'member_register_password' ,'member_coppa_form' ,'mentionme_popup' ,'accountswitcher_header_userbit' ,'accountswitcher_header_dropdown_userbit' ,'accountswitcher_header' ,'accountswitcher_header_dropdown' ,'accountswitcher_header_accountlist' ,'accountswitcher_newpm_messagebit' ,'accountswitcher_newpm_message' ,'accountswitcher_footer' ,'accountswitcher_post_userbit' ,'accountswitcher_post' ,'accountswitcher_sec_accountsbit' ,'accountswitcher_shared_accountsbit' ,'accountswitcher_sidebar_userbit' ,'accountswitcher_sidebar' ,'accountswitcher_avatar' ,'accountswitcher_profile_switch' ,'accountswitcher_profile_link' ,'accountswitcher_profile' ,'myalerts_headericon' ,'myalerts_modal' ,'myalerts_popup_row' ,'myalerts_alert_row_no_alerts' ,'myalerts_js_popup' ,'thankyoulike_member_profile' ,'headerinclude' ,'header' ,'footer' ,'gobutton' ,'htmldoctype' ,'header_welcomeblock_member' ,'header_welcomeblock_member_user' ,'header_welcomeblock_member_moderator' ,'header_welcomeblock_member_admin' ,'error' ,'global_pending_joinrequests' ,'global_awaiting_activation' ,'nav' ,'nav_sep' ,'nav_bit' ,'nav_sep_active' ,'nav_bit_active' ,'footer_languageselect' ,'footer_themeselect' ,'global_unreadreports' ,'footer_contactus' ,'global_boardclosed_warning' ,'global_bannedwarning' ,'error_inline' ,'error_nopermission_loggedin' ,'error_nopermission' ,'global_pm_alert' ,'header_menu_search' ,'header_menu_portal' ,'redirect' ,'footer_languageselect_option' ,'video_dailymotion_embed' ,'video_facebook_embed' ,'video_liveleak_embed' ,'video_metacafe_embed' ,'video_myspacetv_embed' ,'video_veoh_embed' ,'video_vimeo_embed' ,'video_yahoo_embed' ,'video_youtube_embed' ,'debug_summary' ,'smilieinsert_row' ,'smilieinsert_row_empty' ,'smilieinsert' ,'smilieinsert_getmore' ,'smilieinsert_smilie' ,'global_board_offline_modal' ,'footer_themeselector' ,'task_image' ,'usercp_themeselector_option' ,'php_warnings' ,'mycode_code' ,'mycode_email' ,'mycode_img' ,'mycode_php' ,'mycode_quote_post' ,'mycode_size_int' ,'mycode_url' ,'global_no_permission_modal' ,'global_boardclosed_reason' ,'nav_dropdown' ,'global_remote_avatar_notice' ,'header_welcomeblock_member_pms' ,'header_welcomeblock_member_search' ,'header_welcomeblock_guest' ,'header_menu_calendar' ,'header_menu_memberlist' ,'global_dst_detection' ,'header_quicksearch' ,'smilie') AND sid IN ('-2','-1','2') ORDER BY sid ASC</message>
</error>
I was really hoping something in the updates would fix the issue but it seems not
