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UPDATE: I reformulated my question to stress that my focus is on the influence of customs checks, not on the violence in Northern Ireland in general.

My impression is that the EU and most British supporters of a soft Brexit agree that any solution that involves customs border between Ireland and Northern Ireland is dangerous. Is it?

According to my experience, over 95% of private traffic on the customs border between Sweden and Norway passes through without stopping. I assume that a similar arrangement could be used on the Irish border.

Is there evidence that irregular spot-checking of a small minority of private traffic by customs officers would provoke violence?

Notes:

I'm sure that military posts on the border could easily become targets of violence, which is something that happened during The Troubles, but this is very different from hardly noticeable customs posts that let the great majority of private traffic pass through (which is the case on the border between Sweden and Norway).

I know that Northern Ireland is more violent than the European average, as shown by the murder of Lyra McKee, but this does not prove that lightweight customs checks would lead to increased violence.

New here, but there's a misconception in your question. The worry over the border isn't about some trade disputes and companies turning ugly over tariffs; this is about the establishment of any border at all between 2 regions of Ireland.

I definitely can't go into all the details but here's a reference to the Wikipedia on The Troubles which is central to this conversation. From there, some simple Google foo will let you find more information on the violence that was happening within a generation ago over this very border. The Good Friday Agreement is a good place to start as well, although there is debate over whether that prevents trade checkpoints or ALL checkpoints. See other questions on this very site.

And as for evidence of violence well.... This happened just a few days ago

To expand on Red Mage's answer, Wikipedia has a timeline of the Northern Ireland Troubles and peace process. To better understand it, it's good to note that the Good Friday Agreement was effective from December 1999.

Since the Wikipedia list is chronological, a good way to visualise the violence is to use the search option in your browser (which adds little yellow stripes in the scrollbar where the search term occurs).

For example, when we search for the word 'shot', we see many more instances before the turn of the millennium than after it. Other frequent terms include 'bomb', 'explosive', 'killed' and 'wound'. Notice that each of these terms are unlikely to be associated with other EU borders, it's not something you'd expect on the peaceful continent of Europe.

Another good way to read into the situation is to follow up on Wikipedia's references. The link to more extensive news articles which add a lot of context. It should also be noted that the list seems properly maintained. For example, the shooting of the journalist around the 2019 Easter weekend is not yet listed on that page because it's not yet confirmed to be related to the Troubles (though as explained in Red Mage's link, the police says the 'New IRA' is likely to be behind it).

Regarding the updated question:

Customs posts were regularly targeted by the IRA throughout the Troubles as they viewed them as symbolic of the division between Northern Ireland and the Irish Republic. [...]

The Irish Prime Minister brought a copy of an Irish newspaper edition to the [EU-27] summit dinner to emphasise how far Northern Ireland and Ireland had come since the Troubles. The article shared the story of an IRA bombing at the Newry Customs Office in 1972. Nine people were killed and six injured in the attack when a bomb exploded prematurely on August 22. It was the worst attack on a customs post during the Troubles and the single biggest loss of life of customs officials (four). [...]

Another IRA attack on Cloghoge checkpoint near Newry in May 1992 resulted in the death of one soldier and the wounding of 23 others. [...]

PSNI Chief Constable George Hamilton warned earlier this year that if border posts and security installations were created as a result of a hard Brexit that they would be seen as "fair game" for attack by dissident republicans.

“The last thing we would want is any infrastructure around the border because there is something symbolic about it and it becomes a target for violent dissident republicans," the police chief said.

“Our assessment is that they would be a target because it would be representative of the state and in their minds fair game for attack. I would assume that that assessment is shared by senior politicians and officials who are negotiating Brexit."

The Chairman of the Police Federation for Northern Ireland Mark Lindsay said that border posts would make "sitting ducks" out of police.

“If we are saying in the future that police officers could be deployed to customs posts and other fixed points on a hardened border then they would become static targets. They would in effect become sitting ducks for the terrorists,” he said.

As you can see from the above mini-timeline, attack on the initially unhardened customs post(s) is what led to their militarization during the Troubles. The (militarized) border posts that you see in iconic imagery of the later Troubles didn't just happen to be there from the beginning. It was part of PIRA's "Long War" strategy to

To make the Six Counties... ungovernable except by colonial military rule.