all the complete post can be read here
https://www.phpbb.com/community/viewtopic.php?f=556&t=2405081

We have an MS architecture, IIS 8.5, MS SQL 2008 R2, Windows 2012 R2.
So, I replicated this specifications on another host to test the upgrade from 3.0.12 to 3.2

everytime I tried I get this error

General Error:Index name 'phpbb_attachments_PK_phpbb_attachments' on table 'phpbb_attachments' is too long. The maximum is 30 characters. in file C:\inetpub\wwwroot\mv-agusta.es\foro\phpbb\db\tools\tools.php on line 1561BACKTRACEFILE: (not given by php)LINE: (not given by php)CALL: installer_msg_handler()FILE: [ROOT]/phpbb/db/tools/tools.phpLINE: 1561CALL: trigger_error()FILE: [ROOT]/phpbb/db/tools/mssql.phpLINE: 527CALL: phpbb\db\tools\tools->check_index_name_length()FILE: [ROOT]/phpbb/db/tools/mssql.phpLINE: 625CALL: phpbb\db\tools\mssql->sql_create_unique_index()FILE: [ROOT]/phpbb/db/tools/tools.phpLINE: 507CALL: phpbb\db\tools\mssql->sql_column_change()FILE: (not given by php)LINE: (not given by php)CALL: phpbb\db\tools\tools->perform_schema_changes()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 711CALL: call_user_func_array()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 658CALL: phpbb\db\migrator->run_step()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 400CALL: phpbb\db\migrator->process_data_step()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 351CALL: phpbb\db\migrator->try_apply()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 351CALL: phpbb\db\migrator->try_apply()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 351CALL: phpbb\db\migrator->try_apply()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 288CALL: phpbb\db\migrator->try_apply()FILE: [ROOT]/phpbb/db/migrator.phpLINE: 241CALL: phpbb\db\migrator->update_do()FILE: [ROOT]/phpbb/install/module/update_database/task/update.phpLINE: 160CALL: phpbb\db\migrator->update()FILE: [ROOT]/phpbb/install/module_base.phpLINE: 149CALL: phpbb\install\module\update_database\task\update->run()FILE: [ROOT]/phpbb/install/installer.phpLINE: 219CALL: phpbb\install\module_base->run()FILE: [ROOT]/phpbb/install/controller/update.phpLINE: 133CALL: phpbb\install\installer->run()FILE: (not given by php)LINE: (not given by php)CALL: phpbb\install\controller\update->phpbb\install\controller{closure}()FILE: [ROOT]/vendor/symfony/http-foundation/StreamedResponse.phpLINE: 113CALL: call_user_func()FILE: [ROOT]/vendor/symfony/http-foundation/Response.phpLINE: 374CALL: Symfony\Component\HttpFoundation\StreamedResponse->sendContent()FILE: [ROOT]/install/app.phpLINE: 57CALL: Symfony\Component\HttpFoundation\Response->send()

I tried it, to convert from 3.0.12 to 3.1.10 and the result was OK, tried againt from 3.1.10 to 3.2 and the error happens again.
Also tried from 3.0.12 to 3.2, same luck.
then I changed the SQL Server, also tried an SQL Server 2008 R2 Express and a 2014 version, none worked.

so I guess that there is a bug with 3.2 with MSSQL SGBD