Can an employee who concluded an employment agreement with flexible work hours be also employed somewhere else?

Yes, an employee can work at another job.

By entering into an employment contract with non-fixed working hours, an employee is not obliged to give up the opportunity to work elsewhere. It is important that:

• An employer with whom an agreement with non-fixed working hours has been concluded has no right to prohibit an employee from engaging in other work, unless such activities may affect the performance of basic labor duties or violate corporate rules (for example, a conflict of interest).
• Any additional obligations or changes in working conditions should be agreed upon in advance and included in a written contract.