I've found that by disabling Enhanced Security, this has resolved my issues.

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown]
"bEnhancedSecurityStandalone"=dword:00000000
"bProtectedMode"=dword:00000000

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\cDefaultExecMenuItems]
"tWhiteList"="Close|GeneralInfo|Quit|FirstPage|PrevPage|NextPage|LastPage|ActualSize|FitPage|FitWidth|FitHeight|SinglePage|OneColumn|TwoPages|TwoColumns|ZoomViewIn|ZoomViewOut|ShowHideBookmarks|ShowHideThumbnails|Print|GoToPage|ZoomTo|GeneralPrefs|SaveAs|FullScreenMode|OpenOrganizer|Scan|Web2PDF:OpnURL|AcroSendMail:SendMail|Spelling:Check Spelling|PageSetup|Find|FindSearch|GoBack|GoForward|FitVisible|ShowHideArticles|ShowHideFileAttachment|ShowHideAnnotManager|ShowHideFields|ShowHideOptCont|ShowHideModelTree|ShowHideSignatures|InsertPages|ExtractPages|ReplacePages|DeletePages|CropPages|RotatePages|AddFileAttachment|FindCurrentBookmark|BookmarkShowLocation|GoBackDoc|GoForwardDoc|DocHelpUserGuide|HelpReader|rolReadPage|HandMenuItem|ZoomDragMenuItem|CollectionPreview|CollectionHome|CollectionDetails|CollectionShowRoot|&Pages|Co&ntent|&Forms|Action &Wizard|Recognize &Text|P&rotection|&Sign && Certify|Doc&ument Processing|Print Pro&duction|Ja&vaScript|&Accessibility|Analy&ze|&Annotations|D&rawing Markups|Revie&w"

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\cDefaultFindAttachmentPerms]
"tSearchAttachmentsWhiteList"="3g2|3gp|3gpp|3gpp2|aac|ac3|aif|aiff|ani|asf|avi|bmp|cdr|cur|divx|djvu|doc|docx|dv|emf|eps|flv|f4v|gif|ico|iff|jbig2|jp2|jpeg|jpg|m2v|m4a|m4b|m4p|m4v|mid|mkv|mov|mpa|mp2|mp3|mp4|mts|nsv|ogg|ogm|ogv|pbm|pgm|png|ppm|ppt|pptx|ps|psd|qt|rtf|riff|svg|tif|ts|txt|ram|rm|rmvb|vob|wav|wma|wmf|wmv|xmb|xls|xlsx"

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\cDefaultLaunchAttachmentPerms]
"tBuiltInPermList"="version:1|.ade:3|.adp:3|.app:3|.arc:3|.arj:3|.asp:3|.bas:3|.bat:3|.bz:3|.bz2:3|.cab:3|.chm:3|.class:3|.cmd:3|.com:3|.command:3|.cpl:3|.crt:3|.csh:3|.desktop:3|.dll:3|.exe:3|.fxp:3|.gz:3|.hex:3|.hlp:3|.hqx:3|.hta:3|.inf:3|.ini:3|.ins:3|.isp:3|.its:3|.job:3|.js:3|.jse:3|.ksh:3|.lnk:3|.lzh:3|.mad:3|.maf:3|.mag:3|.mam:3|.maq:3|.mar:3|.mas:3|.mat:3|.mau:3|.mav:3|.maw:3|.mda:3|.mdb:3|.mde:3|.mdt:3|.mdw:3|.mdz:3|.msc:3|.msi:3|.msp:3|.mst:3|.ocx:3|.ops:3|.pcd:3|.pi:3|.pif:3|.prf:3|.prg:3|.pst:3|.rar:3|.reg:3|.scf:3|.scr:3|.sct:3|.sea:3|.shb:3|.shs:3|.sit:3|.tar:3|.taz:3|.tgz:3|.tmp:3|.url:3|.vb:3|.vbe:3|.vbs:3|.vsmacros:3|.vss:3|.vst:3|.vsw:3|.webloc:3|.ws:3|.wsc:3|.wsf:3|.wsh:3|.z:3|.zip:3|.zlo:3|.zoo:3|.pdf:2|.fdf:2|.jar:3|.pkg:3|.tool:3|.term:3|.acm:3|.asa:3|.aspx:3|.ax:3|.ad:3|.application:3|.asx:3|.cer:3|.cfg:3|.chi:3|.class:3|.clb:3|.cnt:3|.cnv:3|.cpx:3|.crx:3|.der:3|.drv:3|.fon:3|.gadget:3|.grp:3|.htt:3|.ime:3|.jnlp:3|.local:3|.manifest:3|.mmc:3|.mof:3|.msh:3|.msh1:3|.msh2:3|.mshxml:3|.msh1xml:3|.msh2xml:3|.mui:3|.nls:3|.pl:3|.perl:3|.plg:3|.ps1:3|.ps2:3|.ps1xml:3|.ps2xml:3|.psc1:3|.psc2:3|.py:3|.pyc:3|.pyo:3|.pyd:3|.rb:3|.sys:3|.tlb:3|.tsp:3|.xbap:3|.xnk:3|.xpi:3|.air:3|.appref-ms:3|.desklink:3|.glk:3|.library-ms:3|.mapimail:3|.mydocs:3|.sct:3|.search-ms:3|.searchConnector-ms:3|.vxd:3|.website:3|.zfsendtotarget:3"

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\cDefaultLaunchURLPerms]
"tSchemePerms"="version:2|shell:3|hcp:3|ms-help:3|ms-its:3|ms-itss:3|its:3|mk:3|mhtml:3|help:3|disk:3|afp:3|disks:3|telnet:3|ssh:3|acrobat:2|mailto:2|file:1|rlogin:3|javascript:4|data:3|jar:3|vbscript:3"
"tSponsoredContentSchemeWhiteList"="http|https"
"tFlashContentSchemeWhiteList"="http|https|ftp|rtmp|rtmpe|rtmpt|rtmpte|rtmps|mailto"

Just ran into this issue but it was not with signing documents. It was just trying to edit them. found that the new Adobe Acrobat UI is not working properly. I switched back to the old UI by clicking the Menu, and disabling new Acrobat. Then no more issues.

I had trouble with various tricks, but to my surprise, re-installing Acrobat resolved the issue. Now, all the tools remain in the menu without disappearing when I click on something.

Try compatibility..right click icon then compatibility 

Hey,

I found a solution for this. Control panel_Uninstall a program_change_slect first circle from the window_Ok

This worked for me

is there any update on this issue on why this happens? we are effected with this issue and we cannot turn off Antivirus in enterprise.

Appreciate if anyone can help with the same

I have been having the same problem. Very behind with having documents done as keep hoping that an update will fix the problem. 
I'm surprised there has been no response.