Blocks disappear with block Translation Module activated

subscribing.
way too many hours spent so far with no results

Check your logs. If there are errors with "Illegal mix of collations" for i18n_blocks and more you have a problem with differences in collation. Set them to the same thing. I used the DB Tuner module to set all tables to "utf8_general_ci".

I have a similar problem, when block translation module activated, blocks disappear. But my logs say that there's a problem with the request to mysql:

preg_match() [function.preg-match]: Compilation failed: internal error: previously-checked referenced subpattern not found at offset 389 in /usr/local/www/apache22/data/gelu-tur/includes/database.inc on line 347.

and

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'LEFT JOIN i18n_blocks i18n ON (b.module = i18n.module AND b.delta = i18n.delta) ' at line 1 query: SELECT DISTINCT b.* FROM blocks b LEFT JOIN blocks_roles r ON b.module = r.module AND b.delta = r.delta WHERE b.theme = 'garland' AND b.status = 1 AND (r.rid IN (2) OR r.rid IS NULL) LEFT JOIN i18n_blocks i18n ON (b.module = i18n.module AND b.delta = i18n.delta) WHERE (i18n.language ='en' OR i18n.language ='' OR i18n.language IS NULL) ORDER BY b.region, b.weight, b.module in /usr/local/www/apache22/data/gelu-tur/modules/block/block.module on line 456.

The possible root of evil and a temporary solution can be found here

It seems the other issue has more information so marking this one as duplicate.

See #1268858: When Block translation module is enabled, blocks and menus are not displayed