I received the same error this morning but since it still was before breakfast, I was just briefly using my Smart Phone, otherwise, if I had started my computer I obviously could have taken a screenshot, but it was definitely a Cloudflare error, so scrolling down the Cloudflare 5xx Server Error Overview Page, if memory serves me right, it was a 521 Web Server is down error message. I wish I could say that with certainty, but FWIW, the amount of information in the error message corresponds with that one.
Now if someone with a better memory knows better, and that may well be, s/he can provide more specific information, but for the record, this is not the first time this has happened, but anyway, the host server is usually up and running after a couple of hours, so at least this could help you investigate the error. And maybe Cloudflare could provide you with more information (if they keep logs, of course).
MS Win 10 Pro 64-bit, EAM Pro/EEK, MB 3 Free, WPP, SWB Free, CryptoPrevent Free, NVT OSA and Unchecky, WFW, FFQ with CanDef, uBO, Ghostery, Grammarly Free and HTTPS Ew. Acronis TI 2018, K. Sw. Upd. AM-tools: 9-lab RT BETA, AdwCleaner, Auslogics AM, aswMBR, Avira PCC, BD ART, catchme, Cezurity AV, CCE, CKS, ClamWin P., Crystal Sec., DDS, DWCI, EMCO MD, eScan MWAV, ESS/EOS, FGP, FMTB, FRST, F-SOS, FSS, FreeFixer, GMP, GMER, hP BETA, HJT, Inherit, JRT, K. avz4, KVRT, K. TDSSKiller, LSP-Fix, MB 3 Free, MBAR BETA, MA Stinger, NMC, NoBot, NPE, NSS, NVT MRF (NMRF), OTL, PCC, QD, RCS, RSIT, RKill, Rs, SC, SR, SAP, SVRT, SAS, SL, TMHC, TSA ART, UHM, Vba32 AR, VRS, WR (AiO), Xvirus PG, ZAM, ZHPC, ZHPD and Zoek. I have 23 Years of PC Experience. Bold = effective.