I think you should properly configure the workers for your servers. This error almost certainly due to the fact that you have already had too many threads running within your python process, and due to a resource limit of some kind the request to create a new thread is refused.

Configuration sample

• Server with 4 CPU, 8 Thread
• 60 concurrent users
• 60 users / 6 = 10 <- theoretical number of worker needed
• (4 * 2) + 1 = 9 <- theoretical maximal number of worker
• We’ll use 8 workers + 1 for CRON. We’ll also use a monitoring system to measure CPU load and check if it’s between 7 and 7.5.
• RAM = 9 * ((0.8*150) + (0.2*1024)) ~= 3Go RAM for Odoo

In Odoo.Conf

[options]
limit_memory_hard = 1677721600
limit_memory_soft = 629145600
limit_request = 8192
limit_time_cpu = 600
limit_time_real = 1200
max_cron_threads = 1
workers = 8

For more details Read Odoo Deployment Documentation.

https://www.odoo.com/documentation/11.0/setup/deploy.html#builtin-server