Microsoft Office on Mojave unusable due to sandbox errors

Microsoft Office for years and years has had issues with network shares and Mac clients. There are/have been multiple overlapping issues. The type of problem you are describing is more consistent with using a network share and not local files, your example file path is also consistent with a network share.

1. Office stores temporary files on the same network share in an invisible folder. Permissions on the network share might prevent the creation of this invisible folder and this will then of course cause problems. This folder is called .TemporaryItems and will be a the root/top of the network share, based on your example it would therefore be at /Volumes/myshare/.TemporaryItems on the file server you should check this invisible folder exists and has the right permissions of rwxrwxrwx
2. Even if the folder exists it will then contain sub-folders one for each user, these sub-folders will be named with the uidNumber of the respective user. The problem here is that if you are using purely local user accounts on the client Mac these will not match the uidNumber of the file server or directory server. It is even possible if not likely that multiple Mac clients each with different local account names might all have the same uidNumber as the Mac operating system starts on a single Mac creating them at 501 and increments from there. Therefore two Macs each with one account might end up using the same 501 number. This can result in two users trying to use the same temporary sub-folder. The way to avoid this is to have a directory server, to have each Mac 'bound' to that directory server, to configure it so that it automatically creates Mobile Accounts on the client Macs. The Mobile Accounts on the client Macs work just like a local account but synchronise their credentials to the directory server and this also ensures each user has a matching uidNumber on the client Mac to what their directory server account is and hence completely solves this particular problem.
3. When accessing files on a file server programs are supposed to use routines to properly read and write and more importantly to detect and manage the possibility of two or more users/programs accessing the same file at the same time. Microsoft have a very poor reputation over this. This in particular means the Office feature allowing multiple users to work at the same time on the same document - especially Excel causes problems.
4. It maybe that using SMB rather than AFP might be better. Apple have (almost) discontinued support for AFP and for several years have been encouraging people to switch to using SMB instead.

Especially if you want multiple users to work on the same document at the same time then a web-browser based approach is far more reliable. This could be Google Docs with Google Drive, or it could be Office via a web-browser using Office365. See - https://support.office.com/en-us/article/quick-access-to-your-office-files-in-the-browser-dc1024b4-92be-46eb-81a7-aea85368baa0

If you have not already done so, also ask in the forums run by the maker of those apps, e.g.

https://answers.microsoft.com/en-us/msoffice/forum/msoffice_word-mso_mac