Here it looks like a problem with 5.5, I wrote in the post above, but it may have a connection.
I tried the test mode and everything went well once, unfortunately I subsequently started a sharp operation and the e-shop listed a problem with the payment and there is this in the console:
Object
ba_client_id: "BP-3oqonMsOgJ-B0"
ba_partner_ids: {adverticum: "", mediaMath: ""}
ba_vid: "4425b828-f39e-42d3-b192-ca5212941c41"
event_method: "init"
event_name: "addBarionPixelId"
ga: "GA1.2.912337375.1602493146"
gid: "GA1.2.104124901.1602493146"
iid: "d0babea7-df65-42d1-84e5-a850122b6f68"
init_avh: 1040
init_avw: 1920
init_base_uri: "/pokladna/"
init_bh: 2174
init_bw: 1903
init_cd: 24
init_domain: "robothexo.cz"
init_lang: "cs-CZ"
init_plat: "Win32"
init_ref: "https://robothexo.cz/kosik/"
init_sh: 1080
init_sw: 1920
init_tzo: -2
init_ua: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.75 Safari/537.36"
init_url: "https://robothexo.cz/pokladna/"
marketingConsent: 0
ord: 0
sessionId: "adf062a3-93c3-4ca0-bb71-ef2d9aeba6d0"
sid: "32871ed3-c1ed-4e7d-a32b-7c52266037e8"
tsClient: 1602622204637
vid: "4425b828-f39e-42d3-b192-ca5212941c41,6e4e1b279fea76c35c07507a436bddbc,1602621916794,1602622204636,1602622204636,32871ed3-c1ed-4e7d-a32b-7c52266037e8,1"
__proto__: Object