Tried to delete the folder but I get the same error.
This is the content from the db.log if it could help in finding the problem.
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: SELECT `attrib_value` FROM `observium_attribs` WHERE `attrib_type` = 'dbSchema';
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.dbSchema' doesn't exist), Query: SELECT `version` FROM `dbSchema` ORDER BY `version` DESC LIMIT 1
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: SELECT `attrib_value` FROM `observium_attribs` WHERE `attrib_type` = 'current_rev';
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: SELECT `attrib_value` FROM `observium_attribs` WHERE `attrib_type` = 'current_version';
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.eventlog' doesn't exist), Query: INSERT INTO `eventlog` (`device_id`,`entity_id`,`entity_type`,`timestamp`,`severity`,`message`) VALUES ('0',NULL,'global',NOW(),'5','Observium updated: -> 19.8.10000')
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: SELECT EXISTS (SELECT 1 FROM `observium_attribs` WHERE `attrib_type` = 'current_rev')
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: INSERT INTO `observium_attribs` (`attrib_type`,`attrib_value`) VALUES ('current_rev','10000')
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: SELECT EXISTS (SELECT 1 FROM `observium_attribs` WHERE `attrib_type` = 'current_version')
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: INSERT INTO `observium_attribs` (`attrib_type`,`attrib_value`) VALUES ('current_version','19.8.10000')
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.config' doesn't exist), Query: SELECT * FROM `config`
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.observium_attribs' doesn't exist), Query: SELECT `attrib_value` FROM `observium_attribs` WHERE `attrib_type` = 'dbSchema';
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.dbSchema' doesn't exist), Query: SELECT `version` FROM `dbSchema` ORDER BY `version` DESC LIMIT 1
[2019/12/15 08:26:46 +0100] discovery.php(371): Failed dbQuery (#1146 - Table 'observium.devices' doesn't exist), Query: SELECT 1 FROM `devices` LIMIT 1;