Important : You must backup all your existing Grupo files & database before applying any of these fix. We are not responsible for the loss of any data.

Showing Incorrect Encoding or Invalid Characters
This is usually due to wrong DB collation. Kindly Check your database Collation.

Make sure your database collation is utf8mb4_unicode_ci
Run the following SQL statements

UPDATE gr_defaults SET v2=convert(cast(convert(v2 using  latin1) as binary) using utf8)  WHERE 1;
UPDATE gr_phrases SET full=convert(cast(convert(full using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_phrases SET short=convert(cast(convert(short using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_profiles SET name=convert(cast(convert(name using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_profiles SET v1=convert(cast(convert(v1 using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_profiles SET v2=convert(cast(convert(v2 using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_users SET name=convert(cast(convert(name using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_users SET email=convert(cast(convert(email using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_options SET v1=convert(cast(convert(v1 using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_options SET v2=convert(cast(convert(v2 using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_options SET v3=convert(cast(convert(v3 using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_options SET v4=convert(cast(convert(v4 using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_msgs SET msg=convert(cast(convert(msg using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_msgs SET rtxt=convert(cast(convert(rtxt using  latin1) as binary) using utf8) WHERE 1;
UPDATE gr_permissions SET name=convert(cast(convert(name using  latin1) as binary) using utf8) WHERE 1;

Visit <yourgrupodomain>/act/reset using your web browser & try to login as Admin

Showing undefined/test in header when opening chat
Run the following SQL statement

ALTER TABLE `gr_msgs` ADD `lnurl` TEXT NULL AFTER `cat`, ADD `lntitle` VARCHAR(150) NULL DEFAULT '0' AFTER `lnurl`, ADD `lndesc` VARCHAR(150) NULL DEFAULT '0' AFTER `lntitle`, ADD `lnimg` TEXT NULL AFTER `lndesc`;

Make sure you have set a default timezone in Grupo settings (Menu > Settings).