I'm using a odoo community v.11 instance, with a lot of custom modules.
I have a sale order in draft state, and I delete a line. When I try to save the order, I get an error. The error occured only twice, on different products. Also, when I log in as admin, all goes well, no error.

Can you give me an ideea where to look? Error below:

Traceback (most recent call last):
  File "/home/danila/odoo11/odoo/odoo/fields.py", line 944, in __get__
    value = record.env.cache.get(record, self)
  File "/home/danila/odoo11/odoo/odoo/api.py", line 967, in get
    value = self._data[key][field][record._ids[0]]
KeyError: 2186

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/danila/odoo11/odoo/odoo/http.py", line 651, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/danila/odoo11/odoo/odoo/http.py", line 310, in _handle_exception
    raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
  File "/home/danila/odoo11/odoo/odoo/tools/pycompat.py", line 87, in reraise
    raise value
  File "/home/danila/odoo11/odoo/odoo/http.py", line 693, in dispatch
    result = self._call_function(**self.params)
  File "/home/danila/odoo11/odoo/odoo/http.py", line 342, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/danila/odoo11/odoo/odoo/service/model.py", line 97, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/danila/odoo11/odoo/odoo/http.py", line 335, in checked_call
    result = self.endpoint(*a, **kw)
  File "/home/danila/odoo11/odoo/odoo/http.py", line 937, in __call__
    return self.method(*args, **kw)
  File "/home/danila/odoo11/odoo/odoo/http.py", line 515, in response_wrap
    response = f(*args, **kw)
  File "/home/danila/odoo11/odoo/addons/web/controllers/main.py", line 934, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "/home/danila/odoo11/odoo/addons/web/controllers/main.py", line 926, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/home/danila/odoo11/odoo/odoo/api.py", line 689, in call_kw
    return call_kw_multi(method, model, args, kwargs)
  File "/home/danila/odoo11/odoo/odoo/api.py", line 680, in call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/home/danila/odoo11/odoo/addons/mail/models/mail_thread.py", line 284, in write
    result = super(MailThread, self).write(values)
  File "/home/danila/odoo11/odoo/addons/mail/models/mail_activity.py", line 343, in write
    return super(MailActivityMixin, self).write(vals)
  File "/home/danila/odoo11/odoo/odoo/models.py", line 3109, in write
    self._write(old_vals)
  File "/home/danila/odoo11/odoo/odoo/models.py", line 3339, in _write
    self.recompute()
  File "/home/danila/odoo11/odoo/odoo/models.py", line 4931, in recompute
    vals = {n: rec[n] for n in ns}
  File "/home/danila/odoo11/odoo/odoo/models.py", line 4931, in <dictcomp>
    vals = {n: rec[n] for n in ns}
  File "/home/danila/odoo11/odoo/odoo/models.py", line 4785, in __getitem__
    return self._fields[key].__get__(self, type(self))
  File "/home/danila/odoo11/odoo/odoo/fields.py", line 951, in __get__
    value = record.env.cache.get(record, self)
  File "/home/danila/odoo11/odoo/odoo/api.py", line 967, in get
    value = self._data[key][field][record._ids[0]]
KeyError: 2186