PackagesNotFoundError in Anaconda after conda update

Question

I've installed Anaconda on Ubuntu 18.04 server recently and today, when I installed some new package, a warning has appeared:

==> WARNING: A newer version of conda exists. <==
  current version: 4.6.14
  latest version: 4.7.5

Please update conda by running

    $ conda update -n base -c defaults conda

So, I've executed aforementioned command (inside my r_env environment if that matters) and everything seemed to be fine:

(r_env) user@server:~$ conda update -n base -c defaults conda
Collecting package metadata: done
Solving environment: done

## Package Plan ##

  environment location: /home/user/anaconda3

  added / updated specs:
    - conda


The following packages will be downloaded:

    package                    |            build
    ---------------------------|-----------------
    conda-4.7.5                |           py37_0         3.0 MB
    conda-package-handling-1.3.10|           py37_0         259 KB
    ------------------------------------------------------------
                                           Total:         3.2 MB

The following NEW packages will be INSTALLED:

  conda-package-han~ pkgs/main/linux-64::conda-package-handling-1.3.10-py37_0

The following packages will be UPDATED:

  ca-certificates                               2019.1.23-0 --> 2019.5.15-0
  certifi                                   2019.3.9-py37_0 --> 2019.6.16-py37_0
  conda                                       4.6.14-py37_0 --> 4.7.5-py37_0
  openssl                                 1.1.1b-h7b6447c_1 --> 1.1.1c-h7b6447c_1

The following packages will be DOWNGRADED:

  anaconda                                   2019.03-py37_0 --> custom-py37_0


Proceed ([y]/n)? y

However, after that I cannot install new packages. It says:

Collecting package metadata (current_repodata.json): done
Solving environment: failed
Collecting package metadata (repodata.json): done
Solving environment: failed

PackagesNotFoundError: The following packages are not available from current channels:

  - pkgs/main/linux-64::r-readxl==1.3.1=r36h29659fb_0
  - pkgs/main/linux-64::r-stringi==1.4.3=r36h29659fb_0
 < ...dozen of r-packages...>
  - pkgs/main/linux-64::r-tidyr==0.8.3=r36h29659fb_0
  - pkgs/main/noarch::r-reprex==0.2.1=r36h6115d3f_0

Current channels:

  - https://repo.anaconda.com/pkgs/main/linux-64
  - https://repo.anaconda.com/pkgs/main/noarch
  - https://repo.anaconda.com/pkgs/r/linux-64
  - https://repo.anaconda.com/pkgs/r/noarch

To search for alternate channels that may provide the conda package you're
looking for, navigate to

    https://anaconda.org

and use the search bar at the top of the page.

I tried to downgrade conda to 4.6.14 with no success - the same error appears. Any chance I can fix the problem without reinstalling Anaconda?