I've next to no knowledge of Dutch contract law, but there is no way I would agree to article 14 as written (FWIW, I hold an English Law degree, though it isn't hugely relevant here). Without language restricting "work" to a reasonable scope, Article 14 appears to require written permission for an absurdly wide range of activities:

> Other than with the prior written permission of the Employer, the Employee is prohibited during the term of the Employment Contract to carry out work – either paid or not – of any nature whatsoever, either for himself and/or for third-parties.

For instance, building myself a home server, fixing my parents' laptop, or even helping a friend move house could be construed as "carry[ing] out work [...] of any nature whatsoever", thus requiring prior written permission, which I doubt is the intent. The clause would be much more reasonable, in my opinion, if coupled with something like the following (this obviously isn't legal advice -- I've virtually no knowledge of Dutch contract law [and it's five in the morning!] -- just a rough idea of how the scope could be limited).

> For the purposes of this Article, "work" refers to any activity arsing from or dependant upon any of the Employer's facilities, research, or other assets or activities. It further refers to any activity which may be of use to, or relates directly or indirectly to, any product or service that is produced, sold, developed, researched, or otherwise under consideration by the Employer.

An IP grant could also be added if needed, along the lines of:

> So far as is permitted under Dutch and/or applicable foreign law, the Employer shall own all rights, title, and interests (including, but not limited to, copyrights, (industrial) design rights, patent rights, (semi-conductor) topography rights, plant variety rights, accompanying rights, trade name rights, trade mark rights, database rights) to any activities and inventions arising, directly or indirectly, out of such work (whether or not said work is reliant upon or conducted with any of the Employer's facilities, research, or other assets or activities), unless a written agreement is made with the Employer prior to the work commencing.

Like I mentioned, it's five in the morning here, so it's almost certain I've overlooked something obvious in drafting those (and it isn't legal advice!), but I would be much more willing to agree to a contract that limits the scope to things that are at least somewhat related to the company, especially given the penalty clause (article 21):

> For each infringement of the provisions of [article 14], the Employee will forfeit to the Employer – contrary to the provisions of article 7:650 of the Netherlands Civil Code – an immediately payable penalty of EUR 10,000.- with the addition of EUR 500 for each day (or part of a day) on which the violation continues, without prejudice to the Employer’s right to claim compliance and compensation of damages.

FWIW, I use GitLab for one of my personal projects, and I've had nothing but a positive experience, so I'm certainly not saying that the contract is malicious (nor do I believe it would ever be enforced frivolously/excessively). It just caught my eye whilst scanning through the contract as being excessively broad and something with which I would have a hard time agreeing.