The issue you are experiencing with reconciling bank statements in your Odoo database might be caused by a few different factors. Here are a few things you can try to resolve the issue:

1. Check the server performance: Make sure that your server has enough resources (memory, CPU, disk) to handle the large amount of data you have in your database. If the server is running low on resources, it may cause the "connection lost" error.

2. Check the network connection: Make sure that the network connection between your server and the client is stable and fast. Slow or unreliable network connections can cause the "connection lost" error.

3. Check the configuration: Check the odoo config file, make sure that the limit_memory_soft and limit_memory_hard are set according to your server resources.

4. Check the database: Make sure that the database is in a healthy state. You can check for any corrupted records or missing indexes that may be causing the performance issues. You could also try to optimize the database with a tool like pgAdmin.

5. Check the module: Make sure that the module you are using to reconcile the bank statements is up-to-date and compatible with your version of Odoo.

6. Check the log files: Check the log files of your Odoo server for any error messages related to the reconciliation process. This can give you more insight into the cause of the issue.

7. Ask for help: If you continue to experience issues with reconciling bank statements, you can seek help from the Odoo community or a professional consultant.

It's also worth noting that reconciling large amount of data can be a time-consuming process, it's a good idea to break the process into smaller chunks and to be patient.